牛客小白月赛13 G（双向搜索）

AC通道
两边同步搜，一步里面A走一次B走两次，遇到对方走过的地方就得到了答案。

#include <bits/stdc++.h>
using namespace std;

const int maxn = 1001;
const int tx[] = {-1, 1, 0, 0, -1, -1, 1, 1};
const int ty[] = {0, 0, -1, 1, -1, 1, -1, 1};
int N, M, Ax, Ay, Bx, By;
bool vis[2][maxn][maxn];
char maze[maxn][maxn];
queue<pair<int, int>> Q[2];

bool bfs(int t) {
    int control = Q[t].size();
    while (control--) {
        auto tmp = Q[t].front(); Q[t].pop();
        for (int i = 0; i < 8 - 4 * t; i++) {
            int x = tmp.first + tx[i];
            int y = tmp.second + ty[i];
            if (x < 0 || x >= N || y < 0 || y >= M || maze[x][y] == '#')
                continue;
            if (vis[1 - t][x][y])    return 1;
            if (!vis[t][x][y])    Q[t].push({x, y}), vis[t][x][y] = 1;
        }
    }
    return 0;
}

int solve() {
    Q[0].push({Ax, Ay});
    Q[1].push({Bx, By});
    vis[0][Ax][Ay] = 1;
    vis[1][Bx][By] = 1;
    int step = 0;
    while (!Q[0].empty() || !Q[1].empty()) {
        step++;
        if (bfs(0))    return step;
        if (bfs(1))    return step;
        if (bfs(1))    return step;
    }
    return -1;
}

int main() {
    scanf("%d %d", &N, &M);
    for (int i = 0; i < N; i++) {
        for (int j = 0; j < M; j++) {
            getchar();
            maze[i][j] = getchar();
            if (maze[i][j] == 'C')    Ax = i, Ay = j;
            if (maze[i][j] == 'D')    Bx = i, By = j;
        }
    }
    int ans = solve();
    if (ans >= 0)    printf("YES
%d
", ans);
    else    puts("NO");
    return 0;
}