LeetCode Maximal Rectangle

题目：

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

class Solution {
public:
	int maximalRectangle(vector<vector<char> > &matrix) {
		int m = matrix.size();
		if (m == 0)
			return 0;
		int n = matrix[0].size();

		//f[i][j]表示第i行从j位置开始连续1的个数，复杂度O(n^2)
		vector<vector<int> > f(m, vector<int>(n, 0));
		for (int i = 0; i < m; i++) {
			f[i][n - 1] = matrix[i][n - 1] == '1' ? 1 : 0;
			for (int j = n - 2; j >= 0; j--) {
				f[i][j] = matrix[i][j] == '1' ? f[i][j + 1] + 1 : 0;
			}
		}
		int ans = 0;
		//复杂度O(n^3)
		for (int i = 0; i < m; i++) {
			for (int j = 0; j < n; j++) {
				int len = f[i][j];
				for (int high = 1; high <= m-i; high++) {
					len = min(f[i + high -1][j], len);
					if (len == 0)
						break;
					else 
						ans = max(ans, len * high);
				}
			}
		}
		return ans;
	}
};