Largest Rectangle

#include<iostream>

using namespace std;

int maxSizeFromHistogram(int a[], int n)
{
    int stack[n];
    int _max = 0;
    int length = 0;
    int tp;
    int areaWithTop;

    int i = 0;
    while (i < n) {
        if (length == 0 || stack[length] <= a[i]) {
            stack[length] = i; length++; i++;
        }
        else {
            tp = stack[length - 1]; length--;
            int w = (length == 0) ? i : i - stack[length - 1] - 1;
            areaWithTop = a[tp] * w;

            if (_max < areaWithTop)
                _max = areaWithTop;
        }
    }

    while (length != 0) {
        tp = stack[length - 1]; length--;
        int w = (length == 0) ? i : i - stack[length - 1] - 1;
        areaWithTop = a[tp] * w;

        if (_max < areaWithTop)
            _max = areaWithTop;
    }
}

int main()
{
    int n; cin >> n;
    int a[n];
    for (int i = 0; i < n; i++)
        a[i] = 0;
    int _max = 0;
    for (int rowIndex = 0; rowIndex < n; rowIndex++)
    {
        for (int j = 0; j < n; j++)
        {
            int value; cin >> value;
            if (value == 0) a[j]++;
            else a[j] = 0;
        }

        int result = maxSizeFromHistogram(a, n);

        if (_max < result) _max = result;
    }
    cout << "Result: " << _max;
}