【题目描述】Given a 2D binary matrix filled with 0’s and 1’s, find the largest rectangle containing all ones and return its area.
给一个为0或1的矩阵，求全部为1组成的最大矩阵的面积。

【解题思路】计算每一列中的1字组成的竖线能向左扩展多长

【考查内容】贪心

class Solution {
public:
    int maximalRectangle(vector<vector<char> > &matrix) {
        int m = matrix.size();
        if(m == 0) 
            return 0;
        int n = matrix[0].size();
        if(n == 0) 
            return 0;
        vector<int> vec(m,0);
        vector<vector<int>> area(m,vec);
        int maxArea = 0;
        for(int i = 0;i<m;i++){ 
            for(int j = i;j< m;j++){
                if(matrix[j][0] == '0')
                    break;
                area[i][j] = j-i+1;
                if(area[i][j] > maxArea) 
                    maxArea = area[i][j];
            }
        }
        
        for(int k = 1;k<n;k++){
            for(int i = 0;i<m;i++){
                int flag = 0;
                for(int j = i;j< m;j++){
                    if(flag == 1){
                        area[i][j] = 0;
                    }else if(matrix[j][k] == '0') {
                        flag = 1;
                        area[i][j] = 0;
                    }else{
                        area[i][j] += j-i+1;
                        if(area[i][j] > maxArea) 
                            maxArea = area[i][j];
                    }
                }
            }
        }
        return maxArea;
    }
};