poj 1330 Nearest Common Ancestors

Problem

poj.org/problem?id=1330
vjudge.net/contest/80844#problem/C

Meaning

求最近公共祖先。

Note

真 · LCA 模版题。
那就备份一发 LCA 模版（链式前向星存图）

Code

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 10000, LOG_N = 14;

int head[N+1], to[N], nxt[N];

void add_edge(int f, int t, int &sz)
{
    to[sz] = t;
    nxt[sz] = head[f];
    head[f] = sz++;
}

int depth[N+1], pa[N+1][LOG_N+1];

void dfs(int now, int fa, int d)
{
    depth[now] = d;
    pa[now][0] = fa;
    for(int i = head[now]; ~i; i = nxt[i])
        if(to[i] != fa)
            dfs(to[i], now, d + 1);
}

void init(int n, int rt)
{
    dfs(rt, -1, 0);
    for(int k = 0; 1 << k + 1 < n; ++k)
        for(int v = 1; v <= n; ++v)
            if(pa[v][k] < 0)
                pa[v][k+1] = -1;
            else
                pa[v][k+1] = pa[pa[v][k]][k];
}

int lca(int x, int y)
{
    if(depth[x] > depth[y])
        swap(x, y);
    for(int i = 0, j = depth[y] - depth[x]; j; j >>= 1, ++i)
        if(j & 1)
            y = pa[y][i];
    if(x == y)
        return x;
    for(int k = LOG_N - 1; k >= 0; --k)
        if(pa[x][k] != pa[y][k])
        {
            x = pa[x][k];
            y = pa[y][k];
        }
    return pa[x][0];
}

bool son[N+1];

int main()
{
    int T;
    scanf("%d", &T);
    while(T--)
    {
        int n;
        scanf("%d", &n);
        memset(head, -1, sizeof head);
        memset(son, false, sizeof son);
        int sz = 0;
        for(int i = 1, f, t; i < n; ++i)
        {
            scanf("%d%d", &f, &t);
            add_edge(f, t, sz);
            son[t] = true;
        }
        int rt = -1;
        for(int i = 1; i <= n; ++i)
            if(!son[i])
            {
                rt = i;
                break;
            }
        init(n, rt);
        int x, y;
        scanf("%d%d", &x, &y);
        printf("%d\n", lca(x, y));
    }
    return 0;
}