CF425 D. Misha, Grisha and Underground

  我天，这竟然是道最近公共祖先的板子题。代码里利用了我的LCA模板，前向星存图。

 Misha, Grisha and Underground

#define _CRT_SECURE_NO_WARNINGS
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
using namespace std;
const int N = 1e5 + 7, H = 20;
int n, tot; //树的根节点
int head_copy[N], head[N]; //存的边的信息，head_copy[i]表示第i个节点的头指针
int to[N << 1]; //第i条边指向的节点
int nxt[N << 1]; //第i条边的下一个指针
int anc[N][H]; //对于每一个节点v，记录anc[v][k]，表示它向上走pow(2,k)步之后到达的节点
int Stack[N], dep[N];
void dfs(int root) {
    int top = 0;
    dep[root] = 1;
    for (int i = 0; i < H; ++i)
        anc[root][i] = root;
    Stack[++top] = root; //先求出anc[v][0]
    memcpy(head_copy, head, sizeof head);//head为原始的第i个节点的头指针
    while (top) {
        int x = Stack[top];
        if (x != root) {
            for (int i = 1; i < H; ++i) { //再求出其他anc[v][k]
                int y = anc[x][i - 1];
                anc[x][i] = anc[y][i - 1];
            }
        }
        for (int &i = head_copy[x]; ~i; i = nxt[i]) { //这里i为引用，会修改head_copy
            int y = to[i];
            if (y != anc[x][0]) {
                dep[y] = dep[x] + 1;
                anc[y][0] = x;
                Stack[++top] = y;
            }
        }
        while (top && head_copy[Stack[top]] == -1) //==-1,这个-1和初始head有关
            top--;
    }
}
inline void swim(int &x, int k) { //从节点x向上移动k步，并将x赋为新走到的节点
    for (int i = 0; k > 0; ++i) {
        if (k & 1)
            x = anc[x][i];
        k /= 2;
    }
}
int lca(int x, int y) { //寻找x, y的LCA。
    int k;
    if (dep[x] > dep[y]) swap(x, y);
    swim(y, dep[y] - dep[x]); //首先利用swim将x,y调整到同一高度
    if (x == y) return x; //若x和y重合，就是我们要找的LCA
    while (true) { //否则，不断第寻找一个最小的k，使得anc[x][k] = anc[y][k]
        for (k = 0; anc[x][k] != anc[y][k]; ++k);
        if (k == 0)
            return anc[x][0];
        x = anc[x][k - 1];//新的x,y和原来的x,y有相同的LCA
        y = anc[y][k - 1];
    }
    return -1;
}
void init() {
    tot = 0;
    memset(head, -1, sizeof head);
}
void add(int u, int v) {
    to[tot] = v;
    nxt[tot] = head[u];
    head[u] = tot++;
}
int dist(int u, int v) {
    int x = lca(u, v);
    int res = dep[u] + dep[v] - 2 * dep[x];
    return res;
}
int main()
{
    ios::sync_with_stdio(false);
    int q, pi;
    while (cin >> n >> q) {
        init();
        for (int i = 2; i <= n; ++i) {
            cin >> pi;
            add(pi, i);
            add(i, pi);
        }
        dfs(1);
        int a, b, c, m1, m2, m3, m;
        while (q--) {
            cin >> a >> b >> c;
            m1 = lca(a, b), m2 = lca(a, c), m3 = lca(b, c);
            if (m1 == m2) m = m3;
            else if (m1 == m3) m = m2;
            else m = m1;
            cout << 1 + max(dist(m, a), max(dist(m, b), dist(m, c))) << endl;
        }
    }

    return 0;
}

by myorange