【C++】最近公共祖先LCA（Tarjan离线算法）&& 洛谷P3379LCA模板

 1 #include<cstdio>
 2 #include<cstring>
 3 
 4 const int maxn = 5e5 + 10,maxm = 5e5 + 10;
 5 int fa[maxn],fir[maxn],frt[maxn],ans[maxm],n,m,s,tot,id;
 6 bool flag[maxn];
 7 //存边
 8 struct Edge{
 9     int v,next;
10 }e[maxn << 1];
11 //存询问
12 struct Ques{
13     int v,next,qid;
14 }q[maxm << 1];
15 //加边
16 void addEdge(int u,int v){
17     e[tot] = (Edge){v,fir[u]};
18     fir[u] = tot++;
19     e[tot] = (Edge){u,fir[v]};
20     fir[v] = tot++;
21 }
22 //加询问
23 void addQues(int u,int v,int qid){
24     q[id] = (Ques){v,frt[u],qid};
25     frt[u] = id++;
26     q[id] = (Ques){u,frt[v],qid};
27     frt[v] = id++;
28 }
29 //读入优化
30 inline int read(){
31     int sum = 0;
32     char ch = getchar();
33     while(ch < ‘0‘ || ch > ‘9‘) ch = getchar();
34     while(ch >= ‘0‘ && ch <= ‘9‘){sum = sum * 10 + ch - ‘0‘; ch = getchar();}
35     return sum;
36 }
37 //输出优化
38 void write(int x){
39     if(x / 10) write(x / 10);
40     putchar(x % 10 + ‘0‘);
41 }
42 //寻找父亲
43 int find(int x){
44     if(fa[x] != x) fa[x] = find(fa[x]);
45     return fa[x];
46 }
47 //合并
48 void unionn(int x,int y){
49     int fx = find(x),fy = find(y);
50     fa[fx] = fy;
51 }
52 //离线操作(u表示当前点，fa表示该点的前驱点)
53 void dfs(int u,int fa){
54     flag[u] = true;//标记已访问
55     //遍历所有与u相连的点
56     for(int i = fir[u];i != -1;i = e[i].next){
57         int v = e[i].v;
58         //如果v不是u的前驱
59         if(v != fa){
60             dfs(v,u);//遍历下一层
61             //寻找是否有询问关系
62             for(int j = frt[v];j != -1;j = q[j].next){
63                 int k = q[j].v;
64                 if(flag[k])
65                     ans[q[j].qid] = find(k);
66             }
67             unionn(v,u);//连接
68         }
69     }
70 }
71 //主函数
72 int main(){
73     n = read(),m = read(),s = read();
74     memset(fir,-1,sizeof(fir));
75     memset(frt,-1,sizeof(frt));
76     for(int i = 1;i < n;i++){
77         fa[i] = i;
78         int x = read(),y = read();
79         addEdge(x,y);
80     }
81     fa[n] = n;
82     for(int i = 1;i <= m;i++){
83         int x = read(),y = read();
84         addQues(x,y,i);
85     }
86     dfs(s,-1);
87     //因为这里已经给出根了，直接从根开始
88     for(int i = 1;i <= m;i++,putchar(‘\n‘)) write(ans[i]);
89     return 0;
90 }