Given a 2D binary matrix filled with 0's and 1's, find the largest rectangle containing all ones and return its area. http://leetcode.com/onlinejudge#question_85

leetcode_maximal_rectangle.cpp

class Solution {
public:
    int maximalRectangle(vector<vector<char> > &matrix) {
        int n = matrix.size();
        if (0 == n)
            return 0;
        int m = matrix[0].size();
        if (0 == m)
            return 0;
            
        int **f;
        f = new int*[n];
        for (int i = 0; i < n; ++i)
            f[i] = new int[m];
            
        for (int i = 0; i < n; ++i)
            for (int j = 0; j < m; ++j)
                f[i][j] = matrix[i][j] - '0' + (j > 0 ? f[i][j - 1] : 0);
                
        int sz = 0;
        int ret = 0;
        for (int k = 0; k < m; ++k) {
            sz = (m - k) * n;
            if (sz <= ret)
                break;
            for (int t = m - 1; t >= k; --t) {
                sz = (t - k + 1) * n;
                if (sz <= ret)
                    break;
                int beg = -1;
                int end = -1;
                for (int i = 0; i < n; ++i)
                    if (f[i][t] - (k > 0 ? f[i][k - 1] : 0) == t - k + 1) {
                        if (beg == -1)
                            beg = i;
                        end = i;
                    }
                    else {
                        if (beg != -1)
                            ret = max(ret, (t - k + 1) * (end - beg + 1));
                        beg = -1;
                    }
                if (beg != -1)
                    ret = max(ret, (t - k + 1) * (end - beg + 1));
            }
        }
        
        for (int i = 0; i < n; ++i)
            delete[] f[i];
        delete[] f;
        
        return ret;
    }
};

can you provide explanation