【LCA】JZOJ_3054 祖孙询问

题意

给出一棵树，询问两个节点的关系。

思路

$LCA$模板。

代码

#include<cstdio>
#include<cstring>
#include<algorithm>

const int t = 16;
int n, m, root, tot;
int f[40001][17], dfn[40001];
int head[40001], next[40001], ver[40001];

void add(int u, int v) {
	ver[++tot] = v;
	next[tot] = head[u];
	head[u] = tot;
}

void dfs(int p, int father) {
	dfn[p] = dfn[father] + 1;
	f[p][0] = father;
	for (int i = 1; i <= t; i++)
		f[p][i] = f[f[p][i - 1]][i - 1];
	for (int i = head[p]; i; i = next[i])
		dfs(ver[i], p);
}

int LCA(int x, int y) {
	if (dfn[x] > dfn[y]) std::swap(x, y);
	if (x == y) return x;
	for (int i = t; i >= 0; i--)
		if (dfn[f[y][i]] >= dfn[x]) y = f[y][i];
	if (x == y) return x;
	for (int i = t; i >= 0; i--)
		if (f[x][i] != f[y][i]) {
			x = f[x][i];
			y = f[y][i];
		}
	return f[x][0];
}

int main() {
	scanf("%d", &n);
	for (int i = 1; i <= n; i++) {
		int father, son;
		scanf("%d %d", &son, &father);
		if (father == -1) {
			root = son;
			continue;
		}
		add(father, son);
	}
	dfs(root, root);
	scanf("%d", &m);
	for (int i = 1; i <= m; i++) {
		int x, y;
		scanf("%d %d", &x, &y);
		int z = LCA(x, y);
		if (z == x) printf("1\n");
		else if (z == y) printf("2\n");
		else printf("0\n");
	}
}