【tarjan+lca】有机化学之神偶尔会做作弊

 P2783 有机化学之神偶尔会做作弊

我觉得我对的一比可就是wa三个点我决定不改了毕竟我已经领会到精髓了

（还有这种人？？？？？

就是tarjan缩点+倍增求lca

因为是无向图， 要判断一下， dfs的时候不能返回上一个点， 否则全图就是一整个强连通分量了qwq

#include<cmath>
#include<cstdio>
#include<iostream>
using namespace std;
const int maxn = 10010, maxm = 50050;
int n, m, q, log2n, num = 0, cot = 0, top = 0, tim = 0;
int head[maxm], hear[maxm], dep[maxn], sd[maxn], jump[maxn][23], sta[maxn], low[maxn], dfn[maxn], x[maxm], y[maxm];
bool vis[maxn];
struct edg {
    int nxt, to;
}e[maxm << 1], edge[maxm << 1];
void add(int from, int to) {
    e[++num].to = to;
    e[num].nxt = head[from];
    head[from] = num;
}
void readd(int from, int to) {
    edge[++cot].to = to;
    edge[cot].nxt = hear[from];
    hear[from] = cot;
}
void tarjan(int u, int fa) {
    low[u] = dfn[u] = ++tim;
    sta[++top] = u;
    vis[u] = 1;
    for(int i = head[u]; i; i = e[i].nxt) {
        int v = e[i].to;
        if(v != fa) {
            if(!dfn[v]) 
            {
                tarjan(v, u);
                low[u] = min(low[u], low[v]);
            }
            else if(vis[v]) low[u] = min(low[u], dfn[v]);
        }
    }
    if(dfn[u] == low[u]) {
        int y;
        while(y = sta[top--]) {
            sd[y] = u;
            vis[y] = 0;
            if(y == u) break;
        }
    }
}
void dfs(int x) {
    for(int i = hear[x]; i; i = edge[i].nxt) 
    {
        int y = edge[i].to;
        if(y != jump[x][0]) 
        {
            dep[y] = dep[x] + 1;
            jump[y][0] = x;
            dfs(y);
        }
    }
}
void init() {
    for(int i = 1; i <= log2n; i++)
        for(int j = 1; j <= n; j++)
            jump[j][i] = jump[jump[j][i-1]][i-1];
}
int LCA(int x, int y) {
    if(dep[x] < dep[y]) swap(x, y);
    int t = dep[x] - dep[y];
    for(int i = 0; (1<<i) <= n; i++) 
        if(t & (1<<i)) x = jump[x][i];
    if(x == y) return x;
    for(int i = log2n; i >= 0; i--)    {
        if(jump[x][i] != jump[y][i]) {
            x = jump[x][i];
            y = jump[y][i];
        }
    }
    return jump[x][0];
}
void print(int ans) {
    if (!ans) return;
    print(ans >> 1);
    printf("%d", ans & 1);
}
int main() {
    scanf("%d%d", &n, &m);
    log2n = log(n) / log(2) + 1;
    for(int i = 1; i <= m; i++) {
        int u, v;
        scanf("%d%d", &x[i], &y[i]);
        add(x[i], y[i]), add(y[i], x[i]);
    }
    for(int i = 1; i <= n; i++)
        if(!dfn[i]) tarjan(i, i);
    for(int i = 1; i <= m; i++) {
        if(sd[x[i]] != sd[y[i]]){
            readd(sd[x[i]], sd[y[i]]), readd(sd[y[i]], sd[x[i]]);
        }
    }
    jump[1][0] = 0;
    dep[1] = 1;
    dfs(1);
    init();
    scanf("%d", &q);
    for(int i = 1; i <= q; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        int lca = LCA(u, v);
        int ans = dep[u] + dep[v] - 2*dep[lca] + 1;
        print(ans);
        printf("\n");
    }
    return 0;
}