POJ 1330 最近公共祖先LCA(Tarjan离线做法)

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题目链接：http://poj.org/problem?id=1330

题目大意十分明了简单，就是给定一棵树，求某两个子节点的最近公共祖先，如果尚不清楚LCA的同学，可以左转百度等进行学习。

稍微需要注意的是，建树顺序需要按照题目给定的顺序进行，也就是说根被设定成第一个给出的结点，如样例2。

此题网上题解颇多，但是多是使用的邻接表存图，于是我这里采用了边表，不过实质上Tarjan的部分思想都是一样的，均利用了并查集。

AC代码：

#include <cstdio>#include <cstring>#include <iostream>using namespace std;typedef long long LL;#define MAXN 10010int t, n;int headEdge[MAXN], headQuery[MAXN], father[MAXN];int totEdge, totQuery;struct Edge {    int to, next;} edge[MAXN << 1];struct Node {    int to, next, num;} Query[MAXN << 1];struct node {    int u, v, lca;} input[MAXN];void init() {    totEdge = totQuery = 0;    memset(headEdge, -1, sizeof(headEdge));    memset(headQuery, -1, sizeof(headQuery));    memset(father, -1, sizeof(father));}void addEdge(int from, int to) {    edge[totEdge].to = to;    edge[totEdge].next = headEdge[from];    headEdge[from] = totEdge++;}void addQuery(int from, int to, int x) {    Query[totQuery].to = to;    Query[totQuery].num = x;    Query[totQuery].next = headQuery[from];    headQuery[from] = totQuery++;}int find_set(int x) {    if(father[x] == x) return x;    else return father[x] = find_set(father[x]);}void union_set(int x, int y) {    x = find_set(x); y = find_set(y);    if(x != y) father[y] = x;}void Tarjan(int u) {    father[u] = u;    for(int i = headEdge[u]; i != -1; i = edge[i].next) {        int v = edge[i].to;        if(father[v] != -1) continue;        Tarjan(v);        union_set(u, v);    }    for(int i = headQuery[u]; i != -1; i = Query[i].next) {        int v = Query[i].to;        if(father[v] == -1) continue;        input[Query[i].num].lca = find_set(v);    }}int main() {    scanf("%d", &t);    while(t--) {        init();        scanf("%d", &n);        int a, b;        int root;        for(int i = 0; i < n - 1; i++) {            scanf("%d%d", &a, &b);            if (i == 0) root = a;// 记录一下根的位置            addEdge(a, b); addEdge(b, a);        }        //如果需要多个查询，i的循环可以开到m次即可        for(int i = 0; i < 1; i++) {            scanf("%d%d", &a, &b);            input[i].u = a, input[i].v = b;            addQuery(a, b, i); addQuery(b, a, i);        }        Tarjan(root);        printf("%d\n", input[0].lca);    }    return 0;}

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