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Kth largest even number in N intervals

  • Last Updated : 23 Nov, 2021

Given a 2D array arr[] of N ranges, indicating integer intervals from L to R inclusive, and a number K, the task is to find Kth largest even number.

Examples:

Input: arr = {{12, 15}, {3, 9}, {2, 5}, {20, 25}, {16, 20}}, K = 9
Output: 6
Explanation: After merging, the ranges become {{2, 9}, {12, 25}} the, even numbers present in the ranges are {2, 4, 6, 8, 12, 14, 16, 18, 20, 22, 24} and the 9th largest even number is 6

Input: arr = {{2, 4}, {8, 12}}, K = 4
Output: 4

 

Approach: The given problem can be solved by using the approach of merging overlapping Intervals. The idea is to sort the intervals with respect to the lower limit of the range and merge them to remove repetition of the integers. Then the below steps can be followed to solve the problem:

  • If K<=0 or N=0 then return -1
  • Initialize, L = arr[i].first and R = arr[i].second
  • Initialize a variable range, to store even numbers within the range
  • Iterate in a loop from idx to 0
    • If R is odd
      • range = floor((R – L + 1) / 2)
      • If, K > range K =  K – range
      • Else return R – 2 * K + 1
    • If R is even
      • range = ceil((float)(R – L + 1) / 2)
      • If, K > range K =  K – range
      • Else, R – 2 * K + 2
  • Return -1

Below is the implementation of the above approach: 

C++




// C++ implementation for the above approach
#include <bits/stdc++.h>
#include <cmath>
using namespace std;
 
// Function to merge overlapping intervals
// and return the Kth largest even number
int calcEven(vector<pair<int, int> > V,
             int N, int k)
{
 
    // Base Case
    if (k <= 0 || N == 0)
        return -1;
 
    // Sort the intervals in increasing order
    // according to their first element
    sort(V.begin(), V.end());
 
    int i, idx = 0;
 
    // Merging the overlaping intervals
    for (i = 1; i < N; i++) {
 
        // If it overlaps with
        // the previous intervals
        if ((V[idx].second >= V[i].first)
            || (V[i].first == V[idx].second + 1)) {
 
            // Merge previous and current intervals
            V[idx].second = max(V[idx].second,
                                V[i].second);
        }
        else {
 
            // Increment the index
            idx++;
 
            V[idx].second = V[i].second;
            V[idx].first = V[i].first;
        }
    }
 
    // Find Kth largest even number
    for (i = idx; i >= 0; i--) {
 
        // Initialize L and R
        int L = V[i].first;
        int R = V[i].second;
 
        // If R is odd
        if (R & 1) {
 
            // Calculate number of even
            // numbers within the range
            int range = (R - L + 1) / 2;
 
            // if k > range then kth largest
            // even number is not in this range
            if (k > range) {
 
                k -= range;
            }
            else
                return (R - 2 * k + 1);
        }
        else {
 
            // Calculate number of even
            // numbers within the range
            int range = ceil((float)(R - L + 1) / 2);
 
            // if k > range then kth largest
            // even number is not in this range
            if (k > range) {
 
                k -= range;
            }
            else
                return (R - 2 * k + 2);
        }
    }
 
    // No Kth even number in the range
    // so return -1
    return -1;
}
 
// Driver Code
int main()
{
 
    // Initialize the ranges
    vector<pair<int, int> > vec = { { 12, 15 },
                                    { 3, 9 },
                                    { 2, 5 },
                                    { 20, 25 },
                                    { 16, 20 } };
 
    int N = vec.size();
 
    // Initialize K
    int K = 9;
 
    // Print the answer
    cout << calcEven(vec, N, K);
 
    return 0;
}

Java




// Java implementation for the above approach
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
 
class GFG {
 
  static class Pair {
    int first = 0;
    int second = 0;
 
    public Pair(int first, int second) {
      this.first = first;
      this.second = second;
    }
  }
 
  // Function to merge overlapping intervals
  // and return the Kth largest even number
  public static int calcEven(ArrayList<Pair> V, int N, int k) {
 
    // Base Case
    if (k <= 0 || N == 0)
      return -1;
 
    // Sort the intervals in increasing order
    // according to their first element
    Collections.sort(V, new Comparator<Pair>() {
      @Override
      public int compare(Pair p1, Pair p2) {
        return p1.first - p2.first;
      }
    });
 
    int i, idx = 0;
 
    // Merging the overlaping intervals
    for (i = 1; i < N; i++) {
 
      // If it overlaps with
      // the previous intervals
      if ((V.get(idx).second >= V.get(i).first) || (V.get(i).first == V.get(idx).second + 1)) {
 
        // Merge previous and current intervals
        V.get(idx).second = Math.max(V.get(idx).second, V.get(i).second);
      } else {
 
        // Increment the index
        idx++;
 
        V.get(idx).second = V.get(i).second;
        V.get(idx).first = V.get(i).first;
      }
    }
 
    // Find Kth largest even number
    for (i = idx; i >= 0; i--) {
 
      // Initialize L and R
      int L = V.get(i).first;
      int R = V.get(i).second;
 
      // If R is odd
      if ((R & 1) > 0) {
 
        // Calculate number of even
        // numbers within the range
        int range = (R - L + 1) / 2;
 
        // if k > range then kth largest
        // even number is not in this range
        if (k > range) {
 
          k -= range;
        } else
          return (R - 2 * k + 1);
      } else {
 
        // Calculate number of even
        // numbers within the range
        int range = (int) Math.ceil((R - L + 1) / 2);
 
        // if k > range then kth largest
        // even number is not in this range
        if (k > range) {
 
          k -= range;
        } else
          return (R - 2 * k + 2);
      }
    }
 
    // No Kth even number in the range
    // so return -1
    return -1;
  }
 
  // Driver Code
  public static void main(String args[]) {
 
    // Initialize the ranges
    ArrayList<Pair> vec = new ArrayList<Pair>();
 
    vec.add(new Pair(12, 15));
    vec.add(new Pair(3, 9));
    vec.add(new Pair(2, 5));
    vec.add(new Pair(20, 25));
    vec.add(new Pair(16, 20));
 
    int N = vec.size();
 
    // Initialize K
    int K = 9;
 
    // Print the answer
    System.out.println(calcEven(vec, N, K));
  }
}
 
// This code is contributed by gfgking.

Python3




# python implementation for the above approach
import math
 
# Function to merge overlapping intervals
# and return the Kth largest even number
def calcEven(V, N, k):
 
    # Base Case
    if (k <= 0 or N == 0):
        return -1
 
    # Sort the intervals in increasing order
    # according to their first element
    V.sort()
 
    idx = 0
 
    # Merging the overlaping intervals
    for i in range(1, N):
 
        # If it overlaps with
        # the previous intervals
        if ((V[idx][1] >= V[i][0]) or (V[i][0] == V[idx][1] + 1)):
 
            # Merge previous and current intervals
            V[idx][1] = max(V[idx][1], V[i][1])
 
        else:
 
           # Increment the index
            idx += 1
 
            V[idx][1] = V[i][1]
            V[idx][0] = V[i][0]
 
        # Find Kth largest even number
    for i in range(idx, -1, -1):
 
        # Initialize L and R
        L = V[i][0]
        R = V[i][1]
 
        # If R is odd
        if (R & 1):
 
            # Calculate number of even
            # numbers within the rng
            rng = (R - L + 1) // 2
 
            # if k > rng then kth largest
            # even number is not in this rng
            if (k > rng):
                k -= rng
 
            else:
                return (R - 2 * k + 1)
 
        else:
 
            # Calculate number of even
            # numbers within the rng
            rng = math.ceil((R - L + 1) / 2)
 
            # if k > rng then kth largest
            # even number is not in this rng
            if (k > rng):
                k -= rng
 
            else:
                return (R - 2 * k + 2)
 
        # No Kth even number in the rng
        # so return -1
    return -1
 
# Driver Code
if __name__ == "__main__":
 
   # Initialize the ranges
    vec = [[12, 15], [3, 9], [2, 5], [20, 25], [16, 20]]
 
    N = len(vec)
 
    # Initialize K
    K = 9
 
    # Print the answer
    print(calcEven(vec, N, K))
 
    # This code is contributed by rakeshsahni

Javascript




<script>
       // JavaScript Program to implement
       // the above approach
 
       // Function to merge overlapping intervals
       // and return the Kth largest even number
       function calcEven(V, N, k)
       {
 
           // Base Case
           if (k <= 0 || N == 0)
               return -1;
 
           // Sort the intervals in increasing order
           // according to their first element
           V.sort(function (a, b) { return a.first - b.first })
 
           let i, idx = 0;
 
           // Merging the overlaping intervals
           for (i = 1; i < N; i++) {
 
               // If it overlaps with
               // the previous intervals
               if ((V[idx].second >= V[i].first)
                   || (V[i].first == V[idx].second + 1)) {
 
                   // Merge previous and current intervals
                   V[idx].second = Math.max(V[idx].second,
                       V[i].second);
               }
               else {
 
                   // Increment the index
                   idx++;
 
                   V[idx].second = V[i].second;
                   V[idx].first = V[i].first;
               }
           }
 
           // Find Kth largest even number
           for (i = idx; i >= 0; i--) {
 
               // Initialize L and R
               let L = V[i].first;
               let R = V[i].second;
 
               // If R is odd
               if (R & 1) {
 
                   // Calculate number of even
                   // numbers within the range
                   let range = (R - L + 1) / 2;
 
                   // if k > range then kth largest
                   // even number is not in this range
                   if (k > range) {
 
                       k -= range;
                   }
                   else
                       return (R - 2 * k + 1);
               }
               else {
 
                   // Calculate number of even
                   // numbers within the range
                   let range = Math.ceil((R - L + 1) / 2);
 
                   // if k > range then kth largest
                   // even number is not in this range
                   if (k > range) {
 
                       k -= range;
                   }
                   else
                       return (R - 2 * k + 2);
               }
           }
 
           // No Kth even number in the range
           // so return -1
           return -1;
       }
 
       // Driver Code
 
       // Initialize the ranges
       let vec = [{ first: 12, second: 15 },
       { first: 3, second: 9 },
       { first: 2, second: 5 },
       { first: 20, second: 25 },
       { first: 16, second: 20 }];
 
       let N = vec.length;
 
       // Initialize K
       let K = 9;
 
       // Print the answer
       document.write(calcEven(vec, N, K));
 
   // This code is contributed by Potta Lokesh
   </script>
Output
6

Time Complexity: O(N*log N)
Auxiliary Space: O(1)


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