bfs1

class Solution {
public:

    int get_sum(pair<int, int> p) {
        int s = 0;
        while (p.first) {
            s += p.first % 10;
            p.first /= 10;
        }
        while (p.second) {
            s += p.second % 10;
            p.second /= 10;
        }
        return s;
    }

    int movingCount(int threshold, int rows, int cols)
    {
        if (!rows || !cols) return 0;
        queue<pair<int,int>> q;
        bool st[rows][cols];   //这是一个坑！初始化必须要用for两层循环或者memset来！
        memset(st,0,sizeof st);  

        int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};

        int res = 0;
        q.push({0, 0});
        while (q.size()) {
            auto t = q.front();
            q.pop();
            if (st[t.first][t.second] || get_sum(t) > threshold) continue;
            res ++ ;
            st[t.first][t.second] = true;
            for (int i = 0; i < 4; i ++ ) {
                int x = t.first + dx[i], y = t.second + dy[i];
                if (x >= 0 && x < rows && y >= 0 && y < cols) q.push({x, y});
            }
        }

        return res;
    }
};

bfs二

class Solution {
public:

    int get_sum(pair<int, int> p) {
        int s = 0;
        while (p.first) {
            s += p.first % 10;
            p.first /= 10;
        }
        while (p.second) {
            s += p.second % 10;
            p.second /= 10;
        }
        return s;
    }

    int movingCount(int threshold, int rows, int cols)
    {
        if (!rows || !cols) return 0;
        queue<pair<int,int>> q;
        bool st[rows][cols];   //这是一个坑！初始化必须要用for两层循环或者memset来！
        memset(st,0,sizeof st);  

        int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};

        int ans = 1;
        q.push({0, 0});
        st[0][0]=true;
        while (q.size()) {
            auto t = q.front();
            q.pop();
            for (int i = 0; i < 4; i ++ ) {
                int x = t.first + dx[i], y = t.second + dy[i];
                if (x<0||x>=rows||y<0||y>=cols) continue;
                if (st[x][y]) continue;
                if (get_sum({x,y})>threshold) continue;
                ans++;
                st[x][y]=true;
                q.push({x,y});
            }
        }

        return ans;
    }
};