Learn and practice the Tries and Custom Data Structures patterns to solve coding interview problems.

Speedrun the Oracle Coding Interview: Top Problems in Python

Proficiency in advanced data structures is a marker of a highly experienced engineer, and interviewers use such questions to finetune how they grade candidates. To solve problems with very specific requirements that may not be handled by standard data structures, we gain experience in the advanced skill of the design, implementation, and efficient use of custom data structures.

Data Design

# Statement

Implement a data structure that can store multiple values of the same key at different timestamps and retrieve the key's value at a certain timestamp.
 
You'll need to implement the **TimeStamp** class. This class has the following functions:

- **Init():** This function initializes the values dictionary and timestamp dictionary.

- **Set Value(key, value, timestamp):** 
 This function stores the key and value at any given timestamp.
- **Get Value(key, timestamp):**  This function returns the value set for this key at the specified timestamp.
 
> **Note**: When a query requests the value of a key at a timestamp that isn’t recorded, return the value corresponding to the most recent timestamp before the query’s timestamp. If there are no timestamps before the query's timestamp, return an empty string.


**Constraints:**

- $1 \leq$ `key.length`, `value.length` $\leq 20$
- `key` and `value` consist of lowercase English letters and digits.
- $1 \leq$ `timestamp` $\leq 10^3$
- At most $1 \times 10^3$ calls will be made to **Set Value** and **Get Value**.
- All the timestamps, `timestamp`, of **Set Value** are strictly increasing.

# Examples


Given this series of commands, what is the output of the last command?

Init()

Set Value("prediction", "winning", 3)

Get Value("prediction", 2)

Given this series of commands, what is the output of the last command?
Init()
Set Value(“prediction”, “winning”, 3)
Get Value(“prediction”, 2)

Given this series of commands, what is the output of the last command?

Init()

Set Value("prediction", "winning", 3)

Set Value("prediction", "losing", 4)

Set Value("result", "won", 5)

Get Value("prediction", 5)

Given this series of commands, what is the output of the last command?
Init()
Set Value(“prediction”, “winning”, 3)
Set Value(“prediction”, “losing”, 4)
Set Value(“result”, “won”, 5)
Get Value(“prediction”, 5)

Given this series of commands, what is the output of the _last_ command?

Init()

Set Value("prediction", "winning", 3)

Set Value("prediction", "losing", 4)

Get Value("prediction", 3)


Given this series of commands, what is the output of the last command?
Init()
Set Value(“prediction”, “winning”, 3)
Set Value(“prediction”, “losing”, 4)
Get Value(“prediction”, 3)

Time-Based Key-Value Store


Time-Based Key-Value Store

## Try it yourself
 
Implement your solution in `time_stamp.py` in the following coding playground. The supporting code template provided in `binary_search.py` is meant to assist in developing your solution to the problem.

# Template for getting a target value with binary search.

def binary_search(array, target):
 left = 0
 right = len(array) - 1
 while left <= right:
 mid = (left + right) // 2
 if array[mid] == target:
 return mid
 if array[mid] < target:
 left = mid + 1
 else:
 right = mid - 1

class TimeStamp:
    def __init__(self):
        self.values_dict = {}
        self.timestamps_dict = {}

    #  Set TimeStamp data variables
    def set_value(self, key, value, timestamp):

        # Write your code here
        pass

    # Get TimeStamp data variables
    def get_value(self, key, timestamp):

        # Write your code here
        pass


# Quick Fix\nYou are Edward, a Senior Software Engineer who specializes in coding interview patterns, data structures and algorithms, and is great at debugging issues in code.\n\n- Edward, please keep in mind that you do not have to print the data of the placeholders. Also, do not mention anything about yourself.\n\nEdward, you are given the following data in the placeholders:\n- Function name: `__init__, set_value, get_value`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `Python`\n\n- Edward, your task is to understand the type of error that is occurring in the coding language provided.\n- Once you've understood the error as per the error’s description, and why the test case failed, you will suggest how the candidate can improve their solution code to fix the error.\n- Edward, you will not give the corrected code, just a hint on what the error tells, and how to fix it.\n- Edward, please limit your response strictly to 100 words. No more words than the limit.

# Review Code\nEdward, your current task is to review the `__init__, set_value, get_value` function(s) and the other function(s) invoked in it/them.\n\n-Take your time and review the candidate's solution code.\n- Do not review any extra/helper files or functions not invoked in the solution function `__init__, set_value, get_value`.\n- Do not review the code of data structure objects instantiated like a binary tree, linked list, trie, union find, etc., in the solution functions. Just review the solution/algorithm function of the problem. \n- Also, if the solution code doesn’t use the coding pattern stated above, then please encourage the candidate to reattempt the problem using that pattern corectly.

# Review time complexity\n- Please provide a brief time complexity analysis of the function(s) `__init__, set_value, get_value`. \n- While calculating time complexity, include the time complexity of built-in Python functions invoked by the code. \n- The answer should be to the point in approximately 60 words.

# Review space complexity\n- Please provide a brief space complexity analysis of the function(s) `__init__, set_value, get_value`. \n- While calculating space complexity, include the space complexity of built-in Python functions invoked by the code. \n- Do not include the space used for the input parameters and the space used for just returning an output (not used as a scratch space). \n- The answer should be to the point in approximately 60 words.

Convert

TwoDArray

import sys
import json

sys.path.append("../ds_v1")
from data_structure import DataStructure

# throw parsing errors for incorrect test case format
def parsingError(element_string, error_message):
    sys.stderr.write(f"Parsing Error: {element_string} {error_message}")
    exit()


# className: MyCustomDS
class Convert(DataStructure):
    def __init__(self):
        # constructor
        pass

    def __str__(self):
        # override print() for your data structure
        pass

    # return value will be passed to user program
    def construct(self, inp_string: str, type_of_arr: int) -> any:
        # parse test case in json string 'value' to your desired data structure
        parse_list = json.loads(inp_string)
        return parse_list

    # input param contains return value of user program
    def traverse(self, input: any, type_of_arr: int) -> str:
        # convert your data structure 'input' to a json string representing a test case output
        return input


import sys
import json

sys.path.append("../ds_v1")
from data_structure import DataStructure

# throw parsing errors for incorrect test case format
def parsingError(element_string, error_message):
    sys.stderr.write(f"Parsing Error: {element_string} {error_message}")
    exit()


# className: MyCustomDS
class TwoDArray(DataStructure):
    def __init__(self):
        # constructor
        pass

    def __str__(self):
        # override print() for your data structure
        pass

    # return value will be passed to user program
    def construct(self, inp_string: str, type_of_arr: int) -> any:
        # parse test case in json string 'value' to your desired data structure
        parse_list = json.loads(inp_string)
        return parse_list

    # input param contains return value of user program
    def traverse(self, input: any, type_of_arr: int) -> str:
        # TwoDArray your data structure 'input' to a json string representing a test case output
        return input


def evaluate_results(self) -> object:
 """
 self.__inputs is the list of inputs that are passed as input to function
 self.__actual_output is a list of outputs that is produced by the learner's code
 return : self.__edu__result object
 """
 import random

 class TimeStamp:
 def __init__(self):
 self.values_dict = {}
 self.timestamps_dict = {}

 # Set TimeStamp data variables
 def set_value(self, key, value, timestamp):
 if key in self.values_dict:
 if timestamp < self.timestamps_dict[key][- 1]:
 value = self.timestamps_dict[key][- 1]
 elif value != self.values_dict[key][len(self.values_dict[key])-1]:
 self.values_dict[key].append(value)
 self.timestamps_dict[key].append(timestamp)
 else:
 self.values_dict[key] = [value]
 self.timestamps_dict[key] = [timestamp]

 # Find the index of right most occurrence of the given timestamp
 # using binary search
 def search_index(self, n, key, timestamp):
 left = 0
 j=right = n
 mid = 0

 while left < right:
 mid = (left+right) >> 1
 if self.timestamps_dict[key][mid] <= timestamp:
 left = mid + 1
 else:
 right = mid
 return left-1

 # Get time_stamp data variables
 def get_value(self, key, timestamp):
 if key not in self.values_dict:
 return ""
 else:
 index = self.search_index(len(self.timestamps_dict[key]),
 key, timestamp)
 if index > -1:
 return self.values_dict[key][index]

 return ""
 
 def main_eval(input_1, input_2):
 result = []
 ts = TimeStamp()
 result.append(None)
 for i in range(1, len(input_1)):
 if input_1[i] == "get_value":
 res = ts.get_value(input_2[i][0], input_2[i][1])
 result.append(res)
 if input_1[i] == "set_value":
 ts.set_value(input_2[i][0], input_2[i][1], input_2[i][2])
 result.append(None)

 return result

 expected = main_eval(self.__inputs[0], self.__inputs[1])
 # multiple outputs will be added at each index respectively for a single test_case
 expected_outputs = []

 if json.dumps(expected) == json.dumps(self.__actual_outputs[0]):
 expected_outputs.append(expected)
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else:
 expected_outputs.append(expected)
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

 return self.__edu_result

from time_stamp import TimeStamp


def main(input_1, input_2):
    result = []
    ts = TimeStamp()
    result.append(None)
    for i in range(1, len(input_1)):
        if input_1[i] == "get_value":
            res = ts.get_value(input_2[i][0], input_2[i][1])
            result.append(res)
        if input_1[i] == "set_value":
            ts.set_value(input_2[i][0], input_2[i][1], input_2[i][2])
            result.append(None)

    return result



time_based_key_value_store_append_python.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : ["TimeStamp", "set_value", "get_value"]
    - 2 : [[], ["foo", "bar", 1], ["foo", 1]]
    output:
    - 1 : [null, null, "bar"]
  
  - name: "testcase 3"
    inputs:
    - 1 : ["TimeStamp", "set_value", "get_value"]
    - 2 : [[], ["foo", "bar", 1], ["foo", 2]]
    output:
    - 1 :  [null, null, "bar"]
  
  - name: "testcase 4"
    inputs:
    - 1 : ["TimeStamp", "get_value", "set_value"]
    - 2 : [[], ["foo", 1], ["foo", "bar", 1]]
    output:
    - 1 : [null, "", null]
  
  - name: "testcase 5"
    inputs:
    - 1 : ["TimeStamp", "set_value", "set_value", "get_value"]
    - 2 : [[], ["foo", "bar", 1], ["foo", "tar", 4], ["foo", 4]]
    output:
    - 1 : [null, null, null, "tar"]

  - name: "testcase 6"
    inputs:
    - 1 : ["TimeStamp", "set_value", "get_value"]
    - 2 : [[], ["foo", "jar", 6], ["foo", 6]]
    output:
    - 1 : [null, null, "jar"]
    

def sum(input_1, input_2):
    return input_1+input_2

def validate_inputs(self) -> None:
 """
 Validate user-provided test cases for the Time-based key-value store.
 Store the result using the self.store_result(error_msg, status).
 return: void
 """
 function_calls = self.__inputs[0]
 data = self.__inputs[1]

 # Check constraints for the inputs
 if not self.is_valid_2d_array(data):
 self.store_result(("Invalid test case for 2D Array"), ValidationFunction.FAIL)
 elif not self.is_valid_1d_array(function_calls):
 self.store_result(("Invalid test case for 1D Array"), ValidationFunction.FAIL)
 elif function_calls[0] != "TimeStamp":
 self.store_result("The first argument should always be the Constructor's name", ValidationFunction.FAIL)
 elif len(function_calls[1:]) > 1000:
 self.store_result("The number of function calls should be less than or equal to 1000", ValidationFunction.FAIL)
 elif len(data[1:]) > 1000:
 self.store_result("The length of input data should be less than or equal to 1000", ValidationFunction.FAIL)
 else:
 # Validate user-provided test cases
 valid, error_message = self.validate_time_based_store_test_cases(function_calls, data)
 if valid:
 self.store_result("", ValidationFunction.PASS)
 else:
 self.store_result(error_message, ValidationFunction.FAIL)
 
 def is_valid_1d_array(self, arr):
 if not isinstance(arr, list):
 return False

 return all(isinstance(elem, str) and not isinstance(elem, bool) for elem in arr)

 def is_valid_2d_array(self, arr):
 # if not isinstance(arr, list) or not all(isinstance(row, list) for row in arr):
 # return False

 # for row in arr:
 # if any(elem is None or isinstance(elem, list) for elem in row):
 # return False
 # return True

 # return True
 if not isinstance(arr, list) or not arr:
 return False
 for row in arr:
 for element in row:
 if isinstance(element, bool) or element is None:
 return False
 return True


 def validate_time_based_store_test_cases(self, function_calls, data):
 """
 Validate user-provided test cases for Time-based key-value store.
 :param function_calls: List of commands
 :param data: List of command parameters
 :return: A tuple (valid, error_message) where valid is True if the test cases are valid, False otherwise.
 """
 valid_values = {"TimeStamp", "set_value", "get_value"}
 timestamps_set = set()
 timestamp_called = False
 last_timestamp = float('-inf') # Initialize with negative infinity

 if len(function_calls) != len(data):
 return False, "Mismatch in the number of function calls and their corresponding parameters"

 has_set_or_get_after_timestamp = False

 for cmd, args in zip(function_calls, data):
 if cmd not in valid_values:
 return False, f"Invalid command '{cmd}'. Only 'TimeStamp', 'set_value', and 'get_value' are allowed"

 if cmd == "TimeStamp":
 if timestamp_called:
 return False, "Multiple 'TimeStamp' calls are not allowed. Only one 'TimeStamp' call is permitted"
 timestamp_called = True

 # Check if timestamps are strictly increasing
 for timestamp in args:
 if not isinstance(timestamp, int) or not (1 <= timestamp <= 10**3):
 return False, f"Invalid test case TimeStamp"
 if timestamp in timestamps_set or timestamp <= last_timestamp:
 return False, f"Invalid timestamp value: {timestamp}. Timestamps should be strictly increasing"
 timestamps_set.add(timestamp)
 last_timestamp = timestamp

 if data[0] != []:
 return False, "TimeStamp should be empty"

 if cmd == "set_value" and not args:
 return False, "'set_value' command should have exactly three arguments (key, value, timestamp)"

 if cmd == "set_value" or cmd == "get_value":
 has_set_or_get_after_timestamp = True

 if cmd == "set_value":
 if len(args) != 3:
 return False, "'set_value' command should have exactly three arguments (key, value, timestamp)"

 key, value, timestamp = args

 # Check for null values in the set_value command
 if any(value is None for value in args):
 return False, "'set_value' command cannot have null values for keys or values"

 # Ensure key and value are strings
 if not all(isinstance(val, str) for val in [key, value]):
 return False, "Invalid input: 'key' and 'value' should be strings of English letters and digits only"

 # Check if the timestamp is strictly increasing
 timestamp = args[2]

 if timestamp <= last_timestamp:
 return False, f"Invalid timestamp value: {timestamp}. Timestamps should be strictly increasing"
 last_timestamp = timestamp

 key, value, timestamp = args
 if not (1 <= len(str(key)) <= 20 and 1 <= len(str(value)) <= 20):
 return False, f"Invalid length for key or value. Key and value should be between 1 and 20 characters"

 if not all(ch.islower() or ch.isdigit() for ch in str(key)):
 return False, "Key should consist of lowercase English letters and digits only"

 if not all(ch.islower() or ch.isdigit() for ch in str(value)):
 return False, "Value should consist of lowercase English letters and digits only"

 if not isinstance(timestamp, int) or not (1 <= timestamp <= 10**3):
 return False, f"Invalid timestamp value: {timestamp}. Timestamps should be integers in the range [1, 10^3]"

 if timestamp in timestamps_set:
 return False, f"Duplicate timestamp value: {timestamp}. Timestamps should be strictly increasing"
 timestamps_set.add(timestamp)

 elif cmd == "get_value":
 # Check for null values in the get_value command
 if any(value is None for value in args):
 return False, "'get_value' command cannot have null values for keys or timestamps"

 if not args or len(args) != 2 or not isinstance(args[0], str) or not isinstance(args[1], int):
 return False, "Invalid input for 'get_value'"

 if not has_set_or_get_after_timestamp:
 return False, "After 'TimeStamp', there should be a 'set_value' or 'get_value' call"

 return True, ""

Try to solve the Time Based Key-Value Store problem.

Trie

Custom Data Structures

Conclusion

Time-Based Key-Value Store

Statement

Examples

Understand the problem

Figure it out!

Try it yourself