學了一下圓方樹， 好神奇的東西呀。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long

using
 
 namespace std;

const int N = 2e5 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-9;
const double PI = acos(-1);

int n, m, q;

vector<PII> G[N];
vector<int> G1[N];
int dfn[N], low[N], idx, nn;
int stk[N], top;

 
void tarjan(int u, int preid) {
    dfn[u] = low[u] = ++idx;
    stk[++top] = u;
    for(auto& e : G[u]) {
        if(e.se == preid) continue;
        int v = e.fi;
        if(!dfn[v]) {
            tarjan(v, e.se);
            low[u] = min(low[u], low[v]);
            if(low[v] >= dfn[u]) {
                nn
 
++;
                G1[u].push_back(nn);
                G1[nn].push_back(u);
                do {
                    G1[stk[top]].push_back(nn);
                    G1[nn].push_back(stk[top]);
                } while(stk[top--] != v);
            }
        } else low[u] = min(low[u], dfn[v]);
    }
}

int f[N][20], depth[N];

void dfs(int u, int fa) {
    depth[u] = depth[fa] + 1;
    f[u][0] = fa;
    for(int i = 1; i < 20; i++)
        f[u][i] = f[f[u][i - 1]][i - 1];
    for(auto& v : G1[u]) {
        if(v == fa) continue;
        dfs(v, u);
    }
}

int getLca(int u, int v) {
    if(depth[u] < depth[v]) swap(u, v);
    for(int i = 19; i >= 0; i--)
        if(depth[f[u][i]] >= depth[v])
            u = f[u][i];
    if(u == v) return u;
    for(int i = 19; i >= 0; i--)
        if(f[u][i] != f[v][i])
            u = f[u][i], v = f[v][i];
    return f[u][0];
}


int main() {
    scanf("%d%d%d", &n, &m, &q);
    nn = n;
    for(int i = 1; i <= m; i++) {
        int u, v; scanf("%d%d", &u, &v);
        G[u].push_back(mk(v, i));
        G[v].push_back(mk(u, i));
    }
    for(int i = 1; i <= n; i++)
        if(!dfn[i]) tarjan(i, -1);
    dfs(1, 0);
    while(q--){
        int u, v; scanf("%d%d", &u, &v);
        int lca = getLca(u, v);

        printf("%d\n", (depth[u] + depth[v]) / 2 - depth[lca]);
    }
    return 0;
}

/*
*/

Codeforces 1045C Hyperspace Highways (看題解) 圓方樹