倍增求LCA
模版题 https://www.luogu.org/problemnew/show/P3379
讲解 https://www.bilibili.com/video/av41067872/?p=4&t=159
1 #include <bits/stdc++.h> 2 using namespace std; 3 #define _ 0 4 const int maxn = 5e5+5; 5 const int maxbit = 20; 6 vector<int> G[maxn]; 7 int depth[maxn], lg[maxn], father[maxn][maxbit]; 8 void dfs(int nowp, int fa){ 9 depth[nowp] = depth[fa] + 1; 10 father[nowp][0] = fa; 11 for(int j=1; j<=lg[depth[nowp]]+1; j++) 12 father[nowp][j] = father[father[nowp][j-1]][j-1]; 13 for(int i=0; i<G[nowp].size(); i++){ 14 if(G[nowp][i] != fa) 15 dfs(G[nowp][i], nowp); 16 } 17 } 18 int LCA(int u, int v){ 19 if(depth[u] < depth[v]) swap(u, v); 20 //int dc = depth[u]-depth[v]; 21 // for(int j=0; j<maxbit; j++){ 22 // if((dc>>j) & 1) u = father[u][j]; 23 // } 24 while(depth[u] != depth[v]){ 25 u = father[u][lg[depth[u]-depth[v]]]; 26 } 27 if(u==v) return u; 28 for(int j=lg[depth[u]]; j>=0; j--){ 29 if(father[u][j] != father[v][j]){ 30 u = father[u][j]; v=father[v][j]; 31 } 32 } 33 return father[u][0]; 34 } 35 int main(){ 36 lg[0] = -1; 37 for(int i=1; i<maxn; i++) lg[i] = lg[i>>1] + 1; 38 int n, m, s; 39 scanf("%d%d%d", &n, &m, &s); 40 int x, y; 41 for(int i=1; i<n; i++){ 42 scanf("%d%d", &x, &y); 43 G[x].push_back(y); 44 G[y].push_back(x); 45 } 46 dfs(s,0); 47 while(m--){ 48 scanf("%d%d", &x, &y); 49 printf("%d\n", LCA(x,y)); 50 } 51 return (0^_^0); 52 }
