leetcode No221. Maximal Square

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Question

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

Example:

Input: 

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

Output: 4

Algorithm

参照Solution

dp(i,j)=min(dp(i−1,j),dp(i−1,j−1),dp(i,j−1))+1.

Code

这里dp数组比matrix大，是因为方便代码处理，不然需要对matrix[0][0],matrix[1][0],matrix[0][1]做特殊处理。

class Solution {
public:
    int min(int a, int b, int c){
        if(a<b){
            return a<c?a:c;
        }
        return b<c?b:c;
    }
    int maximalSquare(vector<vector<char>>& matrix) {
        if(matrix.empty())
            return 0;
        
        int row = matrix.size();
        int col = matrix[0].size();
        
        vector<vector<int> > dp(row+1, vector<int>(col+1, 0));
        int maxLen = 0;
        for(int i=1;i<=row;i++){
            for(int j=1;j<=col;j++){
                if(matrix[i-1][j-1] == '1'){
                    dp[i][j] = min(dp[i-1][j-1], dp[i-1][j], dp[i][j-1]) + 1;
                        if(maxLen < dp[i][j])
                            maxLen = dp[i][j];
                }
            }
        }
        
        return maxLen * maxLen;
    }
};