BFS（模板）

struct node{
    int x,y,step;
}Node;
node S,E;
int n, m;
char maze[maxn][maxn];
bool flag[maxn][maxn];
int X[4] = {0,0,1,-1};
int Y[4] = {1,-1,0,0};


bool judge(int x, int y){
    if(x >= n || x < 0 || y >= m || y < 0){//邊界約束
        return false;
    }
    if(maze[x][y] == '#' || flag[x][y] == true){//沒碰壁且未被訪問過
        return false;
    }
    return true;
}

int BFS(int x, int y){//返回值是走迷宮最短路
    queue<node> Q;
    Node.x = x, Node.y = y;
    Q.push(Node);
    flag[x][y] = true;
    while(!Q.empty()){
        node top = Q.front();
        Q.pop();
        if(top.x == E.x && top.y == E.y){
            return top.step;
        }
        for(int i = 0; i < 4; i++){//判斷隊首元素相鄰元素
            int tx = top.x + X[i];
            int ty = top.y + Y[i];
            
            if(judge(tx,ty)){
                Node.x = tx;
                Node.y = ty;
                
                Node.step = top.step+1; //記錄層數
                Q.push(Node);
                flag[tx][ty] = true;

            }
        }

    }return -1;//無法到達終點

}