Task: Find the missing number from given an array of N numbers from 1 to N. Example: Input: 1 3 1 4 3 Output: 2 Here is the Code: Program in C++: #include <iostream> using namespace std; int main() { //number of test cases int t; scanf("%d\n",&t); while(t--) { int n; scanf("%d\n",&n); int a[n-1],sum=0; for(int i=0;i<n-1;i++) { scanf("%d\n",&a[i]); sum+=a[i]; } int sn=(n*(n+1))/2; cout<<sn-sum<<endl; } return 0; } Here is the Video: