spoj COT - Count on a tree (树上第K小 LCA+主席树）

#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define mid int m = (l + r) >> 1
const int M = 2e5 + 10;
int p[M][30],dep[M],head[M],sum[M*20],ls[M*20],rs[M*20],root[M*20];
int cnt1,n,idx,cnt,f[M];

struct node {
    int to,next;
}e[M];

void add(int u,int v){
    e[++cnt1].to=v;e[cnt1].next=head[u];head[u]=cnt1;
    e[++cnt1].to=u;e[cnt1].next=head[v];head[v]=cnt1;
}

int lca(int a,int b){
    if(dep[a] > dep[b]) swap(a,b);
    int h = dep[b] - dep[a]; //h为高度差
    for(int i = 0;(1<<i)<=h;i++){  //(1<<i)&f找到h化为2进制后1的位置，移动到相应的位置
        if((1<<i)&h) b = p[b][i];
        //比如h = 5(101),先移动2^0祖先，然后再移动2^2祖先
    }
    //cout<<a<<" "<<b<<endl;
    if(a!=b){
        for(int i = 22;i >= 0;i --){
            if(p[a][i]!=p[b][i]){  //从最大祖先开始，判断a,b祖先，是否相同
                a = p[a][i]; b = p[b][i]; //如不相同，a,b,同时向上移动2^j
            }
        }
        a = p[a][0]; //这时a的father就是LCA
    }
    return a;
}

void build(int l,int r,int &rt){
    rt = ++idx;
    sum[rt] = 0;
    if(l == r) return;
    mid;
    build(l,m,ls[rt]);
    build(m+1,r,rs[rt]);
}

void update(int p,int l,int r,int old,int &rt){
    rt = ++idx;
    ls[rt] = ls[old]; rs[rt] = rs[old]; sum[rt] = sum[old] + 1;
    if(l == r) return ;
    mid;
    if(p <= m) update(p,l,m,ls[old],ls[rt]);
    else update(p,m+1,r,rs[old],rs[rt]);
}

int query(int a,int b,int lc,int cl,int l,int r,int k){
    if(l == r) return l;
    mid;
    int cnt = sum[ls[a]] + sum[ls[b]] - sum[ls[lc]] - sum[ls[cl]];
    if(k <= cnt)
        return query(ls[a],ls[b],ls[lc],ls[cl],l,m,k);
    else
        return query(rs[a],rs[b],rs[lc],rs[cl],m+1,r,k-cnt);
}
int a[M],b[M];

void dfs(int u,int fa){
    f[u] = fa;
    dep[u] = dep[fa] + 1;
    p[u][0] = fa;
    for(int i = 1;i < 20;i ++) p[u][i] = p[p[u][i-1]][i-1];
    update(a[u],1,cnt,root[fa],root[u]);
    for(int i = head[u];i!=-1;i = e[i].next){
        int v = e[i].to;
        if(v == fa) continue;
        dfs(v,u);
    }
}

int main()
{
    int m;
    while(scanf("%d%d",&n,&m)!=EOF){
        cnt1 = 0;
        memset(head,-1,sizeof(head));
        memset(dep,0,sizeof(dep));
        memset(p,0,sizeof(p));
        memset(f,0,sizeof(f));
        for(int i = 1; i <= n;i ++){
            scanf("%d",&a[i]);
            b[i] = a[i];
        }
        idx = 0;
        int l,r,c;
        sort(b+1,b+n+1);
        cnt = unique(b+1,b+1+n)-b-1;
        for(int i = 1;i <= n;i ++)
            a[i] = lower_bound(b+1,b+cnt+1,a[i]) - b;
        for(int i = 1;i <= n-1;i ++){
            scanf("%d%d",&l,&r);
            add(l,r);
        }
        build(1,cnt,root[0]);
        dfs(1,0);
        for(int i = 1;i <= m;i ++){
            scanf("%d%d%d",&l,&r,&c);
            int lc = lca(l,r);
            int id = query(root[l],root[r],root[lc],root[f[lc]],1,cnt,c);
            printf("%d\n",b[id]);
        }
    }
    return 0;
}