Maximal Rectangle

Given a 2D binary matrix filled with 0's and 1's, find the largest rectangle containing only 1's and return its area.

For example, given the following matrix:

1 0 1 0 0
1 0 1 1 1
1 1 1 1 1
1 0 0 1 0

Return 6.

class Solution {
private:
    int largestRectangle(vector<int>& heights){
        stack<int> s;
        heights.push_back(0);
        int res = 0;
        int i = 0;
        while(i < heights.size()){
            if(s.empty() || heights[i] > heights[s.top()]){
                s.push(i);
                i++;
            }else{
                int cur = s.top();
                s.pop();
                if(s.empty()){
                    res = max(res,i*heights[cur]);
                }else{
                    res = max(res,heights[cur]*(i-s.top()-1));
                }
            }
        }
        return res;
    }
public:
    int maximalRectangle(vector<vector<char>>& matrix) {
        if(matrix.empty()) return 0;
        int m = matrix.size();
        int n = matrix[0].size();
        int res = 0;
        vector<int> heights(n,0);
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                heights[j] = matrix[i][j] == '0'? 0 : (1+heights[j]);
            }
            res = max(res,largestRectangle(heights));
        }
       return res;
    }
   
};