Learn and practice the Two Pointers and Modified Binary Search patterns to solve coding interview problems.

Speedrun the Oracle Coding Interview: Top Problems in Python

This module groups together two of the most common patterns that may be used to solve problems where the input data is linear: Two Pointers and Modified Binary Search. Both the Two Pointers and the Modified Binary Search patterns derive their power from their ability to efficiently reduce the size of the search space, that is, both exemplify the divide and conquer approach to problem-solving. After completing this module, we will have the knowledge of how to use this set of related patterns to solve a diverse range of problems.

Two Pointers & Co.

Which triplet is a valid solution for the array `nums` $= [-3, 0, 1, 2, -1, -4]$ that sums to zero?

Which triplet is a valid solution for the array <code>nums</code> <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo><mo stretchy="false">[</mo><mo>−</mo><mn>3</mn><mo separator="true">,</mo><mn>0</mn><mo separator="true">,</mo><mn>1</mn><mo separator="true">,</mo><mn>2</mn><mo separator="true">,</mo><mo>−</mo><mn>1</mn><mo separator="true">,</mo><mo>−</mo><mn>4</mn><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">= [-3, 0, 1, 2, -1, -4]</annotation></semantics></math>=[−3,0,1,2,−1,−4] that sums to zero?

Given the array `nums` $= [0, 0, 0, 0, 0, 0]$, how many unique triplets can you find that sum to zero?

Given the array <code>nums</code> <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo><mo stretchy="false">[</mo><mn>0</mn><mo separator="true">,</mo><mn>0</mn><mo separator="true">,</mo><mn>0</mn><mo separator="true">,</mo><mn>0</mn><mo separator="true">,</mo><mn>0</mn><mo separator="true">,</mo><mn>0</mn><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">= [0, 0, 0, 0, 0, 0]</annotation></semantics></math>=[0,0,0,0,0,0], how many unique triplets can you find that sum to zero?

How many unique triplets sum to zero for the array `nums` $= [3, 5, 7, 9]$?

How many unique triplets sum to zero for the array <code>nums</code> <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo><mo stretchy="false">[</mo><mn>3</mn><mo separator="true">,</mo><mn>5</mn><mo separator="true">,</mo><mn>7</mn><mo separator="true">,</mo><mn>9</mn><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">= [3, 5, 7, 9]</annotation></semantics></math>=[3,5,7,9]?

Given the array `nums` $= [-3, -2, 0, 1, 2, 3]$, which triplet is _not_ a valid solution?

Given the array <code>nums</code> <math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo>=</mo><mo stretchy="false">[</mo><mo>−</mo><mn>3</mn><mo separator="true">,</mo><mo>−</mo><mn>2</mn><mo separator="true">,</mo><mn>0</mn><mo separator="true">,</mo><mn>1</mn><mo separator="true">,</mo><mn>2</mn><mo separator="true">,</mo><mn>3</mn><mo stretchy="false">]</mo></mrow><annotation encoding="application/x-tex">= [-3, -2, 0, 1, 2, 3]</annotation></semantics></math>=[−3,−2,0,1,2,3], which triplet is not a valid solution?

3Sum

3Sum

Implement your solution in the following coding playground.

# 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: `three_sum`\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 `three_sum` 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 `three_sum`. \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) `three_sum`. \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) `three_sum`. \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.

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 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, value: str, type_of_arr: int) -> any:
        # parse test case in json string 'value' to your desired data structure
        parse_input = json.loads(value)
        return parse_input

    # 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
        parse_output = json.dumps(input)
        return parse_output

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
 """
 def three_sum(nums):
 res = []
 nums.sort()
 for i in range(len(nums)):
 if nums[i] > 0:
 break
 if i == 0 or nums[i - 1] != nums[i]:
 twoSumII(nums, i, res)
 return res
 
 def twoSumII(nums, i, res):
 lo, hi = i + 1, len(nums) - 1
 while lo < hi:
 sum = nums[i] + nums[lo] + nums[hi]
 if sum < 0:
 lo += 1
 elif sum > 0:
 hi -= 1
 else:
 res.append([nums[i], nums[lo], nums[hi]])
 lo += 1
 hi -= 1
 while lo < hi and nums[lo] == nums[lo - 1]:
 lo += 1
 
 def sort_2d_array(arr):
 if arr is None:
 return []
 # Sort each inner list
 sorted_inner_lists = [sorted(inner) for inner in arr]
 
 # Sort the 2D list based on the elements of each inner list
 return sorted(sorted_inner_lists, key=lambda x: tuple(x + [float('inf')] * (3 - len(x))))

 

 expected_outputs = [] 
 expected = three_sum(self.__inputs[0])
 expected_outputs.append(expected)

 actual_output = self.__actual_outputs[0]
 if sort_2d_array(actual_output) == sort_2d_array(expected):
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)
 return self.__edu_result

def three_sum(nums):
    
    # Replace this placeholder return statement with your code
    return [[]]

3Sum.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : [-1, 0, 1, 2, -1, -4]
    output: 
    - 1 : [[-1, -1, 2], [-1, 0, 1]]
    
  - name: "testcase 2"
    inputs:
    - 1 : [1, 2, 3, 4, 5]
    output: 
    - 1 : []
    
  - name: "testcase 3"
    inputs:
    - 1 : [0, 0, 0, 0]
    output: 
    - 1 : [[0, 0, 0]]
    
  - name: "testcase 4"
    inputs:
    - 1 : [-3, -2, -1, 0, 1, 2, 3]
    output: 
    - 1 : [[-3, 0, 3], [-3, 1, 2], [-2, -1, 3], [-2, 0, 2], [-1, 0, 1]]
    
  - name: "testcase 5"
    inputs:
    - 1 : [-4, -2, -2, -2, 0, 2, 2, 2, 4]
    output: 
    - 1 : [[-4, 0, 4], [-4, 2, 2], [-2, -2, 4], [-2, 0, 2]]

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

def validate_inputs(self) -> None:
 """
 self.__inputs is the list of inputs that are passed as input to function
 store the result using the self.store_result(error_msg, status)
 return : void
 """
 
 def is_integer_array(arr):
 return all(isinstance(item, int) for item in arr)
 
 def is_within_range(arr, min_value=-1000, max_value=1000):
 return all(min_value <= item <= max_value for item in arr)
 
 arrr = self.__inputs[0]
 if not is_integer_array(arrr) :
 self.store_result ("input array should contain integer values", ValidationFunction.FAIL)
 elif not 3 <= len(arrr) <= 500:
 self.store_result("input length should be within range [3, 500]", ValidationFunction.FAIL)
 elif not is_within_range(arrr):
 self.store_result("Each element of the input array should be within range [-1000, 1000]", ValidationFunction.FAIL)
 else:
 self.store_result("", ValidationFunction.PASS)

Two Pointers

Modified Binary Search

Conclusion

Sum of Three Values

Statement

Examples

Understand the problem

Figure it out!

Try it yourself