221.最大正方形

在一个由 '0' 和 '1' 组成的二维矩阵内，找到只包含 '1' 的最大正方形，并返回其面积。

输入：matrix = [["1","0","1","0","0"],["1","0","1","1","1"],["1","1","1","1","1"],["1","0","0","1","0"]] 输出：4

1. 动规。直接在原数组上维护更新dp表，dp[i][j]表示以matrix[i][j]为正方形右下角端点的最大正方形的边长。则当matrix[i][j]不等于'0'时，发生状态转移方程：

   matrix[i][j]=Math.min(matrix[i-1][j],matrix[i][j-1],matrix[i-1][j-1])+1