倍增法求LCA

 1 #include<iostream>
 2 #include<algorithm>
 3 #include<cstring>
 4 #include<vector>
 5 #include<set>
 6 #include<map>
 7 #include<queue>
 8 using namespace std;
 9 #define INF 0x3f3f3f3f
10 #define M(a, b) memset(a, b, sizeof(a))
11 const int N = 1e4 + 10, M = 20;
12 int deep[N], p[N][25];
13 vector<int> G[N];
14 
15 void dfs(int u, int fa) {    
16     for (int i = 1; i <= M; ++i)
17         p[u][i] = p[p[u][i-1]][i-1];
18     for (int i = 0; i < G[u].size(); ++i) {
19         int v = G[u][i];
20         if (v == fa || deep[v]) continue;
21         deep[v] = deep[u] + 1;
22         p[v][0] = u;
23         dfs(v, u);
24     }
25 }
26 
27 int lca(int a, int b) {
28     if (deep[a] < deep[b]) swap(a, b);
29     for (int det = deep[a]-deep[b], i = 0; det; det >>= 1, ++i)
30         if (det & 1) a = p[a][i];
31     if (a == b) return a;
32     for (int i = M-1; i >= 0; --i) {
33         if (p[a][i] == p[b][i]) continue;
34         a = p[a][i];
35         b = p[b][i];
36     }
37     return p[a][0];
38 }
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posted @ 2017-04-23 10:51  Robin!  阅读(104)  评论(0编辑  收藏