K-th number

• 归并树
• 划分树
• 分块

目前只看会了第一种，在POJ - 2104上跑了接近4s，POJ - 2761直接超时。

发现自己的二分....真的水

归并树：

#include <cstdio>
#include <iostream>
#include <cstring>
#include <algorithm>
#include <queue>

using namespace std;
const int maxn = 1e5 + 10;
const int inf = 0x3f3f3f3f;
int n,m;
int data[maxn];   //初始数据
int d[20][maxn];  //线段树

void build(int deep,int l,int r)    //O(n) 按升序排列的
{
    if(l == r)  { d[deep][l] = data[l] ; return;}
    int mid = l + r >> 1;
    build(deep+1, l ,  mid);
    build(deep+1, mid+1, r);
    //归并排序合并的过程
    int tid = l,i = l, j = mid + 1;
    while(i <= mid && j <= r)
    {
        if(d[deep+1][i] < d[deep+1][j]) d[deep][tid++] = d[deep+1][i++];
        else d[deep][tid++] = d[deep+1][j++];
    }
    while(i <= mid) d[deep][tid++] = d[deep+1][i++];
    while(j <= r) d[deep][tid++] = d[deep+1][j++];
}

int query(int ql,int qr,int val,int l,int r,int deep)
{
    if( r < ql || l > qr) return 0;
    if( ql <= l && r <= qr)
    {
        int num = upper_bound(d[deep] + l, d[deep] + r + 1,val) - (d[deep] + l);
        return num;
    }
    int mid = l+r>>1;
    int ans = 0;
    if(ql <= mid)  ans += query(ql, qr, val, l , mid, deep+1);
    if(qr >  mid)  ans += query(ql, qr, val, mid+1 , r, deep+1);
    return ans;
}

int solve(int ql,int qr,int k)
{
    int l = -inf,r = inf,ans = 0;
    while(l <= r)
    {
        int mid = l + (r-l)/2;
        int num = query(ql, qr, mid, 1 , n , 0);
        if(num >= k) ans = mid,r = mid-1;
        else l = mid+1;
    }
    return ans;
}
void init()
{
    memset(d,0,sizeof(d));
}
int main()
{
    int a,b,c;
    while(~scanf("%d%d",&n,&m))
    {
        init();
        for(int i = 1; i <= n; i++) scanf("%d",&data[i]);
        build(0,1,n);    //建树
        for(int i = 0; i < m; i++)
        {
            scanf("%d%d%d",&a,&b,&c);
            if(a > b) swap(a,b);
            int ans = solve(a,b,c);
            printf("%d\n",ans);
        }
    }
	return 0;
}