Poj1330Nearest Common Ancestors LCA

题意给一颗树，再给一个查询两点之间的最近公共祖先。

#include<iostream>

#include<cstdio>

#include<cstring>

#include<map>

#include<vector>

using namespace std;

const int maxn = 111111;



struct edge

{

    int to; int next;

}e[maxn * 10];

int len;

int head[maxn];

struct Node

{

    int val; int id;

}vis[maxn], dp[maxn][20];

int pos[maxn];

int cnt;

void add(int from, int to)

{

    e[len].to = to;

    e[len].next = head[from];

    head[from] = len++;

}



void dfs(int x, int val)

{

    vis[cnt].val = val; vis[cnt].id = x; pos[x] = cnt++;

    for (int i = head[x]; i != -1; i = e[i].next){

        int cc = e[i].to;

        dfs(cc, val + 1);

        vis[cnt].val = val; vis[cnt].id = x; pos[x] = cnt++;

    }

}



void init(int k)

{

    for (int i = 0; i < k; i++)

        dp[i][0] = vis[i];

    for (int j = 1; (1 << j) <= k; j++){

        for (int i = 0; i + (1 << j) - 1 < k; i++){

            if (dp[i][j - 1].val < dp[i + (1 << (j - 1))][j - 1].val)

                dp[i][j] = dp[i][j - 1];

            else dp[i][j] = dp[i + (1 << (j - 1))][j - 1];

        }

    }

}



int ask(int l, int r)

{

    int k = 0;

    while ((1 << (k + 1)) < r - l + 1) k++;

    if (dp[l][k].val < dp[r - (1 << k) + 1][k].val) return dp[l][k].id;

    else return dp[r - (1 << k) + 1][k].id;

}

int main()

{



    int T; int n;

    cin >> T;

    int root;

    int gg[maxn];

    int a, b;

    while (T--){

        cin >> n;

        len = 0;

        memset(head, -1, sizeof(head));

        for (int i = 1; i <= n; i++) gg[i] = 0;

        for (int i = 0; i<n - 1; i++){

            scanf("%d%d", &a, &b);

            add(a, b); gg[b] = 1;

        }

        for (int i = 1; i <= n; i++) if (!gg[i]) {

            root = i; break;

        }

        cnt = 0;

        dfs(root, 1);

        init(cnt);

        cin >> a >> b;

        int l = pos[a]; int r = pos[b];

        if (l>r) swap(l, r);

        cout << ask(l, r) << endl;

    }

    return 0;

}