C++ program to find length of non empty substring whose sum is even

Suppose we have an array A with n elements. We have to find the length of a non-empty subset of its elements such that their sum is even or return -1 when there is no such subset.

So, if the input is like A = [1, 3, 7], then the output will be 2, because sum of [1, 3] is 4.

Steps

To solve this, we will follow these steps −

n := size of A
for initialize i := 0, when i < n, update (increase i by 1), do:
   if A[i] mod 2 is same as 0, then:
      k := i + 1
if n is 1 AND k is 0, then:
   return -1
otherwise when k is not equal to 0, then:
   return 1
Otherwise
   return 2

Example

Let us see the following implementation to get better understanding −

#include <bits/stdc++.h>
using namespace std;

int solve(vector<int> A) {
   long n = A.size(), k = 0;
   for (long i = 0; i < n; i++) {
      if (A[i] % 2 == 0) {
         k = i + 1;
      }
   }
   if (n == 1 & k == 0) {
      return -1;
   }
   else if (k != 0) {
      return 1;
   } else {
      return 2;
   }
}
int main() {
   vector<int> A = { 1, 3, 7 };
   cout << solve(A) << endl;
}

Input

{ 1, 3, 7 }

Output

2