POJ 2104K-th Number 划分树

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题目：http://poj.org/problem?id=2104

题意：给定一个数组，求给定区间内的第k大值

思路：划分树模板题

#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;const int N = 100100;int n, m;int arr[N], arrs[N];int cut[20][N], seg[20][N];struct node{    int l, r;}s[N*4];void build(int d, int l, int r, int k){    s[k].l = l, s[k].r = r;    if(l == r) return;    int mid = (l + r) >> 1, cnt = mid - l + 1, lb = l, rb = mid + 1;    for(int i = l; i <= mid; i++) /*初始假设左半部分值都等于中间值，然后计算真正等于中间值的个数*/        if(arrs[i] < arrs[mid])            cnt--;    for(int i = l; i <= r; i++)    {        if(i == l) cut[d][i] = 0;        else cut[d][i] = cut[d][i-1];        if(seg[d][i] == arrs[mid])        {            if(cnt) /*扔到左半部分*/                cnt--, cut[d][i]++, seg[d+1][lb++] = seg[d][i];            else seg[d+1][rb++] = seg[d][i]; /*扔到右半部分*/        }        else if(seg[d][i] < arrs[mid])            cut[d][i]++, seg[d+1][lb++] = seg[d][i];        else seg[d+1][rb++] = seg[d][i];    }    build(d + 1, l, mid, k << 1);    build(d + 1, mid + 1, r, k << 1|1);}int query(int d, int l, int r, int k, int x){    if(s[k].l == s[k].r)         return seg[d][s[k].l];    int ls, lss, mid = (s[k].l + s[k].r) >> 1;    if(s[k].l == l) ls = 0, lss = cut[d][r];    else ls = cut[d][l-1], lss = cut[d][r] - ls;    if(x <= lss) return query(d + 1, s[k].l + ls, s[k].l + ls + lss - 1, k << 1, x);    else return query(d + 1, mid + 1 + l - s[k].l - ls, mid + 1 + r - s[k].l - ls - lss, k << 1|1, x - lss);}int main(){    int a, b, c;    while(~ scanf("%d%d", &n, &m))    {        for(int i = 1; i <= n; i++)        {            scanf("%d", &arr[i]);            seg[0][i] = arrs[i] = arr[i];        }        sort(arrs + 1, arrs + 1 + n);        build(0, 1, n, 1);        while(m--)        {            scanf("%d%d%d", &a, &b, &c);            printf("%d\n", query(0, a, b, 1, c));        }    }    return 0;}

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