Find Missing And Repeating

Problem Statement - link #

Given an unsorted array Arr of size N of positive integers. One number ‘A’ from set {1, 2, N} is missing and one number ‘B’ occurs twice in array. Find these two numbers.

Your Task:

You don’t need to read input or print anything. Your task is to complete the function findTwoElement() which takes the array of integers arr and n as parameters and returns an array of integers of size 2 denoting the answer ( The first index contains B and second index contains A.)

Expected Time Complexity: O(N)
Expected Auxiliary Space: O(1)

Examples #

Example 1:

Input:
N = 2
Arr[] = {2, 2}
Output: 2 1
Explanation: Repeating number is 2 and 
smallest positive missing number is 1.

Example 2:

Input:
N = 3
Arr[] = {1, 3, 3}
Output: 3 2
Explanation: Repeating number is 3 and 
smallest positive missing number is 2.

Constraints #

• 2 ≤ N <= 10^5
• 1 ≤ Ai <= N

Solutions #

class Solution{
    public:
    int *findTwoElement(int *arr, int n) {
        unordered_map<int, int> map;
        for(int i = 0; i < n; i++) {
            map[arr[i]]++;
        }
        int *res = new int[2];
        for(int i = 1; i <= n; i++) {
            if(map.find(i) == map.end()) {
                res[1] = i;
            }
            else if(map[i] == 2) {
                res[0] = i;
            }
        }
        return res;
    }
};