Learn and practice the Merge Intervals and K-way Merge patterns to solve coding interview problems.

Grokking the Blind 75 Problems in Java

Many information processing and decision-support problems require us to merge information coming from multiple sources to build up a global picture of a situation before making some business decisions. This module will help us understand and deploy two common coding patterns used to merge lists of structurally homogeneous data to solve hard problems that we encounter, for example, in scheduling, social media applications, and I/O algorithms.

Fusion

# 
# Statement

We are given an input array of meeting time intervals, `intervals`, where each interval has a start time and an end time. Your task is to find the minimum number of meeting rooms required to hold these meetings.

> An important thing to note here is that the specified end time for each meeting is exclusive.


**Constraints**
* $1 <=$ `intervals.length` $<= 10^{3}$
* $0$ $\leq$ $start_{i}$ $\lt$ $end_{i}$ $\leq$ $10^{6}$

# Examples

# Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Meeting time intervals = [ [1, 3], [2, 6], [8, 10], [9, 15], [12, 14] ]

How many meeting rooms are required to hold these meetings?

Meeting time intervals = [ [1, 3], [2, 6], [8, 10], [9, 15], [12, 14] ]
How many meeting rooms are required to hold these meetings?

Meeting time intervals = [ [3, 17], [19, 20], [20, 22], [1, 18], [9, 19], [21, 22], [3, 4], [7, 22] ]

How many meeting rooms are required to hold these meetings?

Meeting time intervals = [ [3, 17], [19, 20], [20, 22], [1, 18], [9, 19], [21, 22], [3, 4], [7, 22] ]
How many meeting rooms are required to hold these meetings?

Meeting time intervals = [ [1, 6], [1, 6], [1, 6], [1, 6], [1, 6], [1, 6], [1, 6], [1, 6] ]

How many meeting rooms are required to hold these meetings?

Meeting time intervals = [ [1, 6], [1, 6], [1, 6], [1, 6], [1, 6], [1, 6], [1, 6], [1, 6] ]
How many meeting rooms are required to hold these meetings?

# Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

> **Note:** As an additional challenge, we have intentionally hidden the solution to this puzzle. 

# Try it yourself

Implement your solution in `MeetingRooms.java` in the following coding playground. You will need the provided supporting code to implement your solution.

import java.util.*;

public class Solution{
    public static int findSets(int[][] intervals) {

        // Replace this placeholder return statement with your code
        return -1;
    }
}

# 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: `findSets`\n- Candidate's submitted solution code: USER_CODE\n- Error: ERROR_DESCRIPTION\n- Failed test case: FAILED_TEST_CASE\n- Coding language: `Java`\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 `findSets` 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 `findSets`.\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) `findSets`. \n- While calculating time complexity, include the time complexity of built-in Java 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) `findSets`. \n- While calculating space complexity, include the space complexity of built-in Java 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 ds_v1.DataStructure.DataStructure;
import java.util.*;
import java.io.IOException;
import com.fasterxml.jackson.databind.ObjectMapper;
public class TwoDArray extends DataStructure {

 // define class attributes
 int [][] arr={{}};
 private void generateError(String data) {
 System.err.println("TwoDArray is not parsable: " + data);
 System.exit(-1);
 }

 public TwoDArray() {
 // constructor
 }
 public TwoDArray(int[][] data) {
 arr = copy(data);
 }

 public String toString() {
 // override toString() for your data structure
 return Arrays.deepToString(arr);
 }

 // return value will be passed to user program
 @Override
 public TwoDArray construct(String new_value, Integer type_of_ds) {
 int[][] data={{}};
 try {
 ObjectMapper mapper = new ObjectMapper();
 data = mapper.readValue(new_value, int[][].class);
 
 } catch (IOException e) {
 e.printStackTrace();
 }
 return new TwoDArray(data);

 }

 // input param contains return value of user program
 @Override
 public String traverse(Object input, Integer type_of_ds) {
 // convert your data structure 'input' to a json string representing a test case output
 return input.toString();
 }
 public static int[][] copy(int[][] src) {
 if (src == null) {
 return null;
 }
 
 int[][] copy = new int[src.length][];
 for (int i = 0; i < src.length; i++) {
 copy[i] = Arrays.copyOf(src[i], src[i].length);
 }
 
 return copy;
 }

}

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 find_sets(intervals):
 import heapq
 # Replace this placeholder return statement with your code

 # If there is no meeting to schedule then no room needs to be allocated.
 if not intervals:
 return 0

 # The heap initialization
 free_rooms = []

 # Sort the meetings in increasing order of their start time.
 intervals.sort(key= lambda x: x[0])

 # Add the first meeting. We have to give a new room to the first meeting.
 heapq.heappush(free_rooms, intervals[0][1])

 # For all the remaining meeting rooms
 for i in intervals[1:]:

 # If the room due to free up the earliest is free, assign that room to this meeting.
 if free_rooms[0] <= i[0]:
 heapq.heappop(free_rooms)

 # If a new room is to be assigned, then also we add to the heap,
 # If an old room is allocated, then also we have to add to the heap with updated end time.
 heapq.heappush(free_rooms, i[1])

 # The size of the heap tells us the minimum rooms required for all the meetings.
 return len(free_rooms)





 # multiple outputs will be added at each index respectively for a single test_case
 expected_outputs = [] 
 # store the 1st expected output that should be displayed to the user 
 expected_outputs.append(find_sets(self.__inputs[0]))

 if json.dumps(find_sets(self.__inputs[0])) == json.dumps(self.__actual_outputs[0]):
 self.__edu_result.store_result(expected_outputs, TestCaseResult.PASS)
 else:
 self.__edu_result.store_result(expected_outputs, TestCaseResult.FAIL)

 return self.__edu_result

import java.util.*;

public class Main{
    public static int minMeetingRooms1(TwoDArray input1) {
        int res = Solution.findSets(input1.arr);
        return res;
    }
}

meeting_rooms_two.yaml

testcases:
  - name: "testcase 1"
    inputs:
    - 1 : [[2, 8], [3, 4], [3, 9], [5, 11], [8, 20], [11, 15]]
    output: 
    - 1 : 3

  - name: "testcase 2"
    inputs:
    - 1: [[1, 3], [2, 6], [8, 10], [9, 15], [12, 14]]
    output:
    - 1 : 2

  - name: "testcase 3"
    inputs:
    - 1 : [[1, 2], [4, 6], [3, 4], [7, 8]]
    output: 
    - 1 : 1

  - name: "testcase 4"
    inputs:
    - 1: [[1, 7], [2, 6], [3, 7], [4, 8], [5, 8]]
    output:
    - 1 : 5

  - name: "testcase 5"
    inputs:
    - 1: [[1, 2], [1, 2], [1, 2]]
    output:
    - 1 : 3

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
 """
 # inputs can be accessed using self.__inputs starting from 0 to len(self.__inputs) - 1
 def is_valid_2d_array(arr):
 if not isinstance(arr,list) or not all(isinstance(row, list) for row in arr):
 return False # Not a valid 2D array if it's empty or contains non-list elements
 
 for row in arr:
 if any(elem is None or isinstance(elem, bool) for elem in row):
 return False # Not a valid 2D array if any element in a row is None
 if not all(isinstance(elem, int) for elem in row):
 return False 
 return True







 
 if not is_valid_2d_array(self.__inputs[0]):
 self.store_result ("Invalid array input", ValidationFunction.FAIL)

 elif not all([len(x)==2 for x in self.__inputs[0]]):

 self.store_result("Length of each interval should be exactly 2", ValidationFunction.FAIL)
 
 
 elif not all(isinstance(start,int) and isinstance(end,int) for start,end in self.__inputs[0]):
 self.store_result ("Intervals should contain integers only", ValidationFunction.FAIL)
 
 

 
 elif not (1 <= len(self.__inputs[0]) <= 10**3):
 self.store_result("Length of input array should be between 1 and 10^3", ValidationFunction.FAIL)


 elif not all((0 <= start < end) for start,end in self.__inputs[0]):
 self.store_result("Start time of each interval should be greater than or equal to 0 and less than its end time", ValidationFunction.FAIL)

 elif not all(end<=10**6 for start,end in self.__inputs[0]):
 self.store_result("End time of each interval should be less than or equal to 10^6", ValidationFunction.FAIL)


 else:
 self.store_result("", ValidationFunction.PASS)

Try to solve the Meeting Rooms II problem.

Merge Intervals

K-way Merge

Conclusion

Meeting Rooms II

Statement

Examples

Understand the problem

Figure it out!

Try it yourself