若对pos先取值,则count函数会失效

    cin >> a >> b;
    // int x = pos[a], y = pos[b]; 这若用pos先取值,cont就没有效果了
    if (pos.count(a) == 0 && pos.count(b) == 0) printf("ERROR: %d and %d are not found.\n", a, b);
    else if (pos.count(a) == 0) printf("ERROR: %d is not found.\n", a);
    else if (pos.count(b) == 0) printf("ERROR: %d is not found.\n", b);
    else{
        // 为啥count之前不能加 取值呢?
        int x = pos[a], y = pos[b];
        while (d[x] != d[y])
        {
            if (d[x] > d[y]) x = p[x];
            else y = p[y];
        }
        // 判断是否相同
        while (x != y)
        {
            x = p[x];
            y = p[y];
        }
        if (in[x] == b) printf("%d is an ancestor of %d.\n", b, a);
        else if (in[x] == a) printf("%d is an ancestor of %d.\n", a, b);
        else printf("LCA of %d and %d is %d.\n", a, b, in[x]);

    }