这个题很久以前就WA了，今天重写一个依然是WA。不知道是哪里错了，希望有人可以帮助debug一下。

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

const int maxn = 2021;
int n, m, q;
map<string, int> id;
string di[maxn];
vector<int> G[maxn];
int depth[maxn], fa[maxn], in[maxn];
char a[maxn], b[maxn];
int pre[maxn];

int find(int x) {
    return x == pre[x] ? x : pre[x] = find(pre[x]);
}

void unite(int x, int y) {
    x = find(x);
    y = find(y);
    if(x != y) pre[x] = y;
}

void dfs(int u, int p, int d) {
    depth[u] = d;
    fa[u] = p;
    for(int i = 0; i < G[u].size(); i++) {
        int &v = G[u][i];
        if(v == u || v == p) continue;
        dfs(v, u, d+1);
    }
}

int query(int u, int v) {
    if(find(u) != find(v)) return -1;
    if(depth[u] < depth[v]) swap(u, v);
    while(depth[u] > depth[v]) u = fa[u];
    while(u != v) {
        u = fa[u];
        v = fa[v];
    }
    return u;
}

int main() {
    // freopen("in", "r", stdin);
    while(~scanf("%d", &n)) {
        id.clear(); m = 0;
        memset(depth, 0, sizeof(depth));
        memset(in, 0, sizeof(in));
        for(int i = 0; i < maxn; i++) {
            pre[i] = i;
            G[i].clear();
        }
        for(int i = 0; i < n; i++) {
            scanf("%s %s", a, b);
            if(id.find(a) == id.end()) {
                id[a] = ++m;
                di[m] = a;
            }
            if(id.find(b) == id.end()) {
                id[b] = ++m;
                di[m] = b;
            }
            G[id[a]].push_back(id[b]);
            unite(id[a], id[b]);
            in[id[b]]++;
        }
        for(int i = 1; i <= m; i++) {
            if(!in[i]) {
                dfs(i, -1, 1);
            }
        }
        scanf("%d", &q);
        while(q--) {
            scanf("%s %s", a, b);
            if(strcmp(a, b) == 0) {
                puts(a);
                continue;
            }
            if(id.find(a) == id.end() || id.find(b) == id.end()) {
                puts("-1");
                continue;
            }
            int ret = query(id[a], id[b]);
            if(ret == -1) puts("-1");
            else puts(di[ret].c_str());
        }
    }
    return 0;
}