221. Maximal Square

    public int maximalSquare(char[][] matrix) {
        int n = matrix.length;
        if(n == 0) return 0;
        int m = matrix[0].length;

        int[][] dp = new int[2][m+1];
        int max = 0;
        int pre = 0;
        int cur = 1;
        for(int i = 1 ; i <= n; ++i){
            for(int j = 1 ; j <= m ; ++j){
                if(matrix[i-1][j-1] == '0'){
                    dp[cur][j] = 0;
                    continue;
                }
                dp[cur][j] = Math.min(Math.min(dp[cur][j-1],dp[pre][j]), dp[pre][j-1]) +1;
                max = Math.max(dp[cur][j], max);

            }
            pre = cur;
            cur = (cur +1)%2;
        }
        return max*max;
    }