Count Pairs With Given Sum

Problem Statement - link #

Given an array of N integers, and an integer K, find the number of pairs of elements in the array whose sum is equal to K.

Your Task:

You don’t need to read input or print anything. Your task is to complete the function getPairsCount() which takes arr[], n and k as input parameters and returns the number of pairs that have sum KS and integer X as input parameters and returns true if students in all rooms can use wifi or false otherwise.

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(N)

Examples #

Example 1:

Input:
N = 4, K = 6
arr[] = {1, 5, 7, 1}
Output: 2
Explanation: 
arr[0] + arr[1] = 1 + 5 = 6 
and arr[1] + arr[3] = 5 + 1 = 6.

Example 2:

Input:
N = 4, K = 2
arr[] = {1, 1, 1, 1}
Output: 6
Explanation: 
Each 1 will produce sum 2 with any 1.

Constraints #

• 1 ≤ N ≤ 10^5
• 1 ≤ K ≤ 10^7
• 1 ≤ A[i] ≤ 10^6

Solutions #

class Solution {
  public:
    int getPairsCount(int arr[], int n, int k) {
        int cnt = 0;
        unordered_map<int,int> mp;
        for(int i = 0; i < n; i++) {
            if(mp[arr[i]] != 0) 
                cnt += mp[arr[i]];
            mp[k-arr[i]]++;
        }
        return cnt;
    }
};