力扣-200题 岛屿数量（C++）- BFS

题目链接：https://leetcode.cn/problems/number-of-islands/
题目如下：

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        //flood-fill算法，要遍历每一个格子，然后分别bfs or dfs
        int res=0;
        int m=grid.size();
        if(m==0) return res;
        int n=grid[0].size();

        vector<vector<bool>> visited(m,vector<bool>(n,false));

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(visited[i][j]==false&&grid[i][j]=='1'){
                    res++;
                    bfs(grid,i,j,visited,m,n);
                }
            }
        }

        return res;
    }
    void bfs(vector<vector<char>> g,int x,int y,vector<vector<bool>>& visited,int m,int n){
        queue<pair<int,int>> que;
        que.push({x,y});
        visited[x][y]=true;

        while(que.size()){
            auto t=que.front();
            que.pop();

            int dx[4]={-1,0,1,0},dy[4]={0,1,0,-1};
            for(int i=0;i<4;i++){
                int a=t.first+dx[i];
                int b=t.second+dy[i];

                if(a>=0&&a<m&&b>=0&&b<n&&visited[a][b]==false&&g[a][b]=='1'){
                    que.push({a,b});
                    visited[a][b]=true;
                }
            }
        }
    }
};