题目链接：POJ-1330
## **题目描述**
In the figure, each node is labeled with an integer from {1, 2,…,16}. Node 8 is the root of the tree. Node x is an ancestor of node y if node x is in the path between the root and node y. For example, node 4 is an ancestor of node 16. Node 10 is also an ancestor of node 16. As a matter of fact, nodes 8, 4, 10, and 16 are the ancestors of node 16. Remember that a node is an ancestor of itself. Nodes 8, 4, 6, and 7 are the ancestors of node 7. A node x is called a common ancestor of two different nodes y and z if node x is an ancestor of node y and an ancestor of node z. Thus, nodes 8 and 4 are the common ancestors of nodes 16 and 7. A node x is called the nearest common ancestor of nodes y and z if x is a common ancestor of y and z and nearest to y and z among their common ancestors. Hence, the nearest common ancestor of nodes 16 and 7 is node 4. Node 4 is nearer to nodes 16 and 7 than node 8 is.
For other examples, the nearest common ancestor of nodes 2 and 3 is node 10, the nearest common ancestor of nodes 6 and 13 is node 8, and the nearest common ancestor of nodes 4 and 12 is node 4. In the last example, if y is an ancestor of z, then the nearest common ancestor of y and z is y.
Write a program that finds the nearest common ancestor of two distinct nodes in a tree.
思路
题目是求最近公共祖先的裸题，然后给定a,b默认a是b的父亲节点；所以我们需要一个并查集来维护一下根节点。然后剩下的就是裸的LCA问题了；
LCA代码
#include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;#define MAX_N 500050
#define MAX_M 500050int head[MAX_N], cnt = 0, fa[MAX_N][30], dfn[MAX_N], pre[MAX_N];
int n, m, t;struct Node{
int to, val, next;
}edge[MAX_M * 2];void init()
{
cnt = 0;memset(head, -1, sizeof(head));memset(dfn, 0, sizeof(dfn));for(int i = 1; i <= n; i++)pre[i] = i;
}int Find(int x)
{
return x == pre[x] ? pre[x] : pre[x] = Find(pre[x]);
}void Union(int x, int y)
{
x = Find(x);y = Find(y);if(x != y)pre[y] = x;
}void add(int x, int y)
{
edge[cnt].to = y;edge[cnt].next = head[x];head[x] = cnt++;
}void dfs(int u)
{
for(int i = head[u]; i != -1; i = edge[i].next){
int v = edge[i].to;if(!dfn[v]){
dfn[v] = dfn[u] + 1;fa[v][0] = u;dfs(v);}}
}int lca(int x, int y)
{
if(dfn[x] < dfn[y]){
swap(x, y);}for(int i = 20; i >= 0; i--){
if(dfn[fa[x][i]] >= dfn[y]){
x = fa[x][i];}}if(x == y)return x;for(int i = 20; i >= 0; i--){
if(fa[x][i] != fa[y][i]){
x = fa[x][i];y = fa[y][i];}}return fa[x][0];
}int main()
{
scanf("%d",&t);while(t--){
scanf("%d", &n);init();for(int i = 0; i < n - 1; i++){
int x, y;scanf("%d %d", &x, &y);Union(x, y);add(x, y);add(y, x);}int root = Find(pre[1]);dfn[root] = 1, fa[root][0] = 0;dfs(root);for(int i = 1; i <= 20; i++){
for(int j = 1; j <= n; j++){
fa[j][i] = fa[fa[j][i-1]][i-1];}}int x, y;scanf("%d %d", &x, &y);printf("%d\n", lca(x, y));}return 0;
}