zoukankan      html  css  js  c++  java
  • [LeetCode 221] Maximal Square

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

    For example, given the following matrix:

    1 0 1 0 0
    1 0 1 1 1
    1 1 1 1 1
    1 0 0 1 0
    

    Return 4.

     

    Solution 1.  For each entry of 1, use it as a possible bottom right point of a square of all 1s, then find the largest square of all 1s for this entry. The runtime is O(n^4): O(n^2) possible entries and O(n^2) time for each check.

    The problem of this solution is that for each entry of 1, we start from scratch to check. We basically throw all the previous check results for previous entries of 1. Take the following example, when checking the entry of red color, we've already checked entries of yellow, green and purple that they all form a length 3 square. However solution 1 discard these infos and checked all the entries in the 4 by 4 grid to determine it actually forms a length 4 square.  Instead, its neighboring results(left, top and top left) can be used to get a O(1) check as shown in solution 2.

    1 1 1 1

    1 1 1 1

    1 1 1 1

    1 1 1 1

     

    Solution 2. Dynamic Programming, O(n^2) runtime, O(n^2) space 

    State: dp[i][j] : the max length of the square of all 1s whose bottom right point is matrix[i][j].

    State Transition: dp[i][j] == 0, if matrix[i][j] == 0;  dp[i][j] = 1 + min{dp[i][j - 1], dp[i - 1][j], dp[i - 1][j - 1]}, if matrix[i][j] == 1.

    Initialization: dp[i][0] = matrix[i][0]; dp[0][j] = matrix[0][j]. 

    class Solution {
        public int maximalSquare(char[][] matrix) {
            if(matrix.length == 0) {
                return 0;
            }
            int n = matrix.length, m = matrix[0].length;
            int[][] dp = new int[n][m]; //dp[i][j]: the max side length of square whose bottom right corner is (i, j).
            int ans = 0;
            for(int i = 0; i < n; i++) {
                for(int j = 0; j < m; j++) {
                    if(matrix[i][j] == '1') {
                        dp[i][j] = 1;
                        if(i > 0 && j > 0) {
                            dp[i][j] = Math.max(dp[i][j], Math.min(Math.min(dp[i - 1][j], dp[i][j - 1]), dp[i - 1][j - 1]) + 1);
                        }
                        ans = Math.max(ans, dp[i][j] * dp[i][j]);
                    }
                }
            }
            return ans;
        }
    }

    Related Problems 

    Maximal Square II

    Maximum Subsquare surrounded by 'X'

  • 相关阅读:
    C#如何用OpenFileDialog控件打开图片显示到PictureBox这个控件
    C# winform 禁止窗体移动
    linux 硬链接和软链接(转)
    linux 源码编译(转)
    linux 压缩与解压缩
    硬盘分区(来自百度百科)
    arp:地址解析协议(Address Resolution Protocol)(来自维基百科)
    c++学习笔记(1)
    ProbS CF matlab源代码(二分系统)(原创作品,转载注明出处,谢谢!)
    [eclipse]UML之AmaterasUML 插件
  • 原文地址:https://www.cnblogs.com/lz87/p/7393778.html
Copyright © 2011-2022 走看看