洛谷P3379 【模板】最近公共祖先（LCA）

鬼知道我为什么要来写这篇博客

诺， 下边的是代码，

具体讲解的话

这里（侵删）

墙裂推荐！

讲的太好了

我才不会说是因为自己懒才推荐别人的，qwq

虽然我的也是看的他的

#include <iostream>
#include <cstdio>
#include <algorithm>
#define N 5000001
using namespace std;
struct exo {
    int t, next;
}e[N];
int dt[N], fa[N][30], lg[N], head[N], cnt, n, m ,s;
void add (int x, int y) {
    e[++cnt].next = head[x];
    e[cnt].t = y;
    head[x] = cnt;
}
void dfs (int f, int fath) {
    dt[f] = dt[fath] + 1;
    fa[f][0] = fath;
    for (int i = 1; (1 << i) <= dt[f]; i++) 
        fa[f][i] = fa[fa[f][i - 1]][i - 1];
    for (int i = head[f];i;i = e[i].next)
        if (e[i].t != fath)
            dfs (e[i].t, f);
}
int lca (int x, int y) {
    if (dt[x] < dt[y]) swap (x, y);
    while (dt[x] > dt[y]) x = fa[x][lg[dt[x] - dt[y]] - 1];
    if (x == y) return x;
    for (int k = lg[dt[x]] - 1; k >= 0;k--) 
        if (fa[x][k] != fa[y][k])
            x = fa[x][k], y = fa[y][k];
    return fa[x][0];
}
int main () {
    scanf ("%d%d%d", &n, &m, &s);
    for  (int i = 1; i < n; i++) {
        int x, y;
        scanf ("%d%d", &x, &y);
        add (x, y);
        add (y, x);
    }
    dfs (s, 0);
    for (int i = 1; i <= n; i++) 
        lg[i] = lg[i - 1] + (1 << lg[i- 1] == i) ;
    for (int i = 1; i <= m; i++) {
        int x, y;
        scanf ("%d%d", &x, &y);
        printf ("%d\n", lca (x, y));
    }
    return 0;
}