LeetCode: Maximal Rectangle

class Solution {
public:

    using pi = pair <int, int>;
    int ar[210][210];
    int up[210][210];

    int maximalRectangle(vector<vector<char>>& matrix) {
        int ans = 0;
        int n = matrix.size();
        if(n == 0) return ans;
        int m = matrix[0].size();
        if(m == 0) return ans;
        for(int i=1;i<=n;i++){
            for(int j=1;j<=m;j++){
                if(matrix[i-1][j-1] == '1'){
                    ar[i][j] = 1;
                    up[i][j] = up[i-1][j] + 1;
                }
                else ar[i][j] = 0;
            }
        }
        for(int i=1;i<=n;i++){
            stack <int> st;
            for(int j=1;j<=m;j++){
                while(!st.empty() and up[i][st.top()] > up[i][j]){
                    int area = 0;
                    int prej = st.top(); st.pop();
                    if(st.empty()) area = up[i][prej] * (j - 1);
                    else area = up[i][prej] * (j - st.top() - 1);
                    ans = max(ans, area);
                }
                st.push(j);
            }
            while(!st.empty()){
                int area = 0;
                int prej = st.top(); st.pop();
                if(st.empty()) area = up[i][prej] * m;
                else area = up[i][prej] * (m - st.top());
                ans = max(ans, area);
            }
        }

        return ans;
    }
};