POJ 2761 Feed the dogs 主席树（区间第k大）

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题目：

http://poj.org/problem?id=2761

题意：

求区间第k大数，很裸

思路：

很裸的主席树

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int N = 100100;int root[N], lson[N*20], rson[N*20], sum[N*20];int a[N], b[N];int n, m, tot;void build(int l, int r, int &rt){    rt = ++tot;    sum[rt] = 0;    if(l == r) return;    int mid = (l + r) >> 1;    build(l, mid, lson[rt]);    build(mid + 1, r, rson[rt]);}void update(int pre, int &rt, int l, int r, int val){    rt = ++tot;    lson[rt] = lson[pre], rson[rt] = rson[pre], sum[rt] = sum[pre] + 1;    if(l == r) return;    int mid = (l + r) >> 1;    if(val <= mid) update(lson[pre], lson[rt], l, mid, val);    else update(rson[pre], rson[rt], mid + 1, r, val);}int query(int ss, int tt, int l, int r, int k){    if(l == r) return l;    int mid = (l + r) >> 1;    int tmp = sum[lson[tt]] - sum[lson[ss]];    if(k <= tmp) return query(lson[ss], lson[tt], l, mid, k);    else return query(rson[ss], rson[tt], mid + 1, r, k - tmp);}int main(){    while(~ scanf("%d%d", &n, &m))    {        for(int i = 1; i <= n; i++) scanf("%d", &a[i]), b[i] = a[i];        sort(b+1, b+1+n);        int len = unique(b+1, b+1+n) - b - 1;        tot = 0;        build(1, len, root[0]);        for(int i = 1; i <= n; i++) a[i] = lower_bound(b+1, b+1+len, a[i]) - b;        for(int i = 1; i <= n; i++) update(root[i-1], root[i], 1, len, a[i]);        int v, u, k;        while(m--)        {            scanf("%d%d%d", &v, &u, &k);            printf("%d\n", b[query(root[v-1], root[u], 1, len, k)]);        }    }    return 0;}

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