[宽度优先搜索] leetcode 542 01 Matrix

problem:https://leetcode.com/problems/01-matrix/

        多源最短路径。这道题之前用深搜做的，复习宽搜的时候又用宽搜刷了一遍，以0为起点即可。

class Solution {
public:
    vector<vector<int>> updateMatrix(vector<vector<int>>& matrix) {
        
        int m = matrix.size();
        int n = matrix[0].size();
        vector<vector<int>> res(m, vector<int>(n, -1));
        queue<int> q;
        for(int i = 0;i < m; i++)
        {
            for(int j = 0; j < n;j++)
            {
                if(matrix[i][j] == 0)
                {
                    res[i][j] = 0;
                    q.push(i * n + j);
                }
            }
        }
        
        vector<int> dx{ 0, 1, 0, -1 };
        vector<int> dy{ 1, 0, -1, 0 };
        while(!q.empty())
        {
            int cur = q.front();
            q.pop();
            
            int i = cur / n;
            int j = cur % n;
            
            for(int k = 0; k < 4; k++)
            {
                int x = i + dx[k];
                int y = j + dy[k];
                if(x >= 0 && y >= 0 && x < m && y < n)
                {
                    if(res[x][y] == -1)
                    {
                        res[x][y] = res[i][j] + 1;
                        q.push(x * n + y);
                    }
                }
            }
        }
        
        return res;
    }
};