Biggest black square algorithmic question

solution.cpp

#include <vector>
#include <stack>
#include <algorithm>
#include <iostream>
using namespace std;

class Solution {
public:
    int solution(vector<int>& A) {
        int N = A.size();
        int maxSide = 0;

        for (int r = 0; r < N; r++) {
            vector<int> heights(N);
            for (int c = 0; c < N; c++) {
                heights[c] = (A[c] >= N - r) ? N - r : 0;
            }
            maxSide = max(maxSide, biggestSquare(heights));
        }

        return maxSide;
    }

private:
    int biggestSquare(vector<int>& heights) {
        stack<int> stk;
        int maxSide = 0;
        int N = heights.size();

        for (int i = 0; i <= N; i++) {
            int h = (i == N) ? 0 : heights[i];
            while (!stk.empty() && heights[stk.top()] > h) {
                int height = heights[stk.top()];
                stk.pop();
                int width = stk.empty() ? i : i - stk.top() - 1;
                maxSide = max(maxSide, min(height, width));
            }
            stk.push(i);
        }

        return maxSide;
    }
};