人丑常数大，总是卡在1000多ms。。。
1 #include <cstdio> 2 #include <cstring> 3 #include <iostream> 4 #define maxn 5000005 5 #define jump_max 19 6 struct Edge{ 7 int from,to; 8 }; 9 Edge edge[maxn]; 10 int n,m,s,x,y,a,b; 11 int dep[maxn]; 12 int f[maxn][20]; 13 int head[maxn]; 14 int tot = 0; 15 inline void swap_ele(int &a,int &b){ 16 int c = a; 17 a = b; 18 b = c; 19 } 20 inline int read(){ 21 int num = 0; 22 char c; 23 bool flag = false; 24 while ((c = getchar()) == ' ' || c == ' ' || c == ' '); 25 if (c == '-') 26 flag = true; 27 else 28 num = c - '0'; 29 while (isdigit(c = getchar())) 30 num = num * 10 + c - '0'; 31 return (flag ? -1 : 1) * num; 32 } 33 void add_edge(int from,int to){ 34 edge[++tot].from = head[from]; 35 edge[tot].to = to; 36 head[from] = tot; 37 } 38 void dfs(int step){ 39 for (int i=head[step];i!=0;i = edge[i].from){ 40 int v = edge[i].to; 41 if (dep[v] == 0){ 42 dep[v] = dep[step] + 1; 43 f[v][0] = step; 44 dfs(v); 45 } 46 } 47 } 48 void init(){ 49 50 for (register int i=1;i<=jump_max;i++) 51 for (register int j=1;j<=n;j++) 52 f[j][i] = f[ f[j][i-1] ][ i-1 ]; 53 } 54 55 int lca(int x,int y){ 56 if (dep[x] < dep[y]) 57 swap_ele(x,y); 58 for (register int i=jump_max;i>=0;--i){ 59 if (dep[f[x][i]] >= dep[y]) 60 x = f[x][i]; 61 } 62 if (x == y) 63 return y; 64 for (register int i=jump_max;i>=0;--i) 65 if (f[x][i] != f[y][i]){ 66 x = f[x][i]; 67 y = f[y][i]; 68 } 69 70 return f[x][0]; 71 } 72 73 int main(){ 74 n = read();m = read();s = read(); 75 for (register int i=1;i<=n-1;++i){ 76 x = read();y = read(); 77 add_edge(x,y);add_edge(y,x); 78 } 79 dep[s] = 1; 80 dfs(s); 81 init(); 82 for (register int i=1;i<=m;++i){ 83 a = read();b = read(); 84 printf("%d ",lca(a,b)); 85 } 86 return 0; 87 }