10032. Find Polygon With the Largest Perimeter

You are given an array of positive integers nums of length n.

A polygon is a closed plane figure that has at least 3 sides. The longest side of a polygon is smaller than the sum of its other sides.

Conversely, if you have k (k >= 3) positive real numbers a1, a2, a3, …, ak where a1 <= a2 <= a3 <= ... <= ak and a1 + a2 + a3 + ... + ak-1 > ak, then there always exists a polygon with k sides whose lengths are a1, a2, a3, …, ak.

The perimeter of a polygon is the sum of lengths of its sides.

Return the largest possible perimeter of a polygon whose sides can be formed from nums, or -1 if it is not possible to create a polygon.

class Solution {
public:
    long long largestPerimeter(vector<int>& A) {
        sort(begin(A), end(A));
        long long res = -1, sum = A[0] + A[1];
        for(int i = 2; i < A.size(); i++) {
            if(sum > A[i]) res = max(res, sum + A[i]);
            sum += A[i];
        }
        return res;
    }
};