POJ 1330 Nearest Common Ancestors LCA

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstring>
 4 #include<vector>
 5 using namespace std;
 6 
 7 const int maxn = 1e4 + 10;
 8 
 9 int n,root;
10 int deg[maxn],vis[maxn],p[maxn];
11 vector<int> tree[maxn],que[maxn];
12 
13 void init() {
14     memset(deg, 0, sizeof(deg));
15     for (int i = 0; i <= n; i++) tree[i].clear();
16     for (int i = 0; i <= n; i++) que[i].clear();
17     memset(vis, 0, sizeof(vis));
18     root = -1;
19 }
20 //并查集：
21 int fin(int x) { return p[x] = p[x] == x ? x : fin(p[x]); }
22 void uni(int a, int b) {
23     int pa = fin(a);
24     int pb = fin(b);
25     //不能写成p[pa]=pb，和tarjan算法有关;
26     p[pb] = pa;
27 }
28 
29 bool tarjan_LCA(int rt,int& ans) {
30     p[rt] = rt;
31     for (int i = 0; i < tree[rt].size(); i++) {
32         int v = tree[rt][i];
33         if (tarjan_LCA(v, ans)) return true;
34         //把v并到rt上
35         uni(rt, v);
36     }
37     //涂黑这一点
38     vis[rt] = 1;
39     //处理与rt有关的查询
40     for (int i = 0; i < que[rt].size(); i++) {
41         int qv = que[rt][i];
42         if (vis[qv]) {
43             ans = fin(qv);
44             return true;
45         }
46     }
47     return false;
48 }
49 
50 int main() {
51     int tc;
52     scanf("%d", &tc);
53     while (tc--) {
54         scanf("%d", &n);
55         init();
56         for (int i = 0; i < n - 1; i++) {
57             int u, v;
58             scanf("%d%d", &u, &v);
59             tree[u].push_back(v);
60             deg[v]++;
61         }
62         int qu, qv;
63         scanf("%d%d", &qu, &qv);
64         que[qu].push_back(qv);
65         que[qv].push_back(qu);
66         for (int i = 1; i <= n; i++) if (deg[i] == 0) {
67             root = i;
68         }
69         int ans;
70         tarjan_LCA(root,ans);
71         printf("%d\n", ans);
72     }
73     return 0;
74 }