Dreamoon and NightMarket Gym - 101234G （二分答案）

题目

https://vjudge.net/problem/Gym-101234G

题意

给你n个数，为你这n个数组成的集合中和第k小的是多少

思路

二分答案

#include <bits/stdc++.h>

using namespace std;
const int maxn = 2e5+100;
const int maxm = 1e6+100;
int a[maxn];
int n,k;
struct node
{
    int num,id;
}sum[maxm];
int ok(int x)
{
    int l,r;
    l = r = 0;
    sum[0].num = 0;
    sum[0].id = 0;
    while(l <= r)
    {
        for(int i = sum[l].id+1;i <= n;i++)
        {
            if(sum[l].num+ a[i] <= x)
            {
                sum[++r].num = sum[l].num + a[i];
                sum[r].id = i;
                if(r >= k) return 1;
            }
            else{
                break;
            }
        }
        l++;
    }
    return 0;
}
int main()
{
    scanf("%d%d",&n,&k);
    for(int i = 1;i <= n;i++)
    {
        scanf("%d",&a[i]);
    }
    sort(a+1,a+1+n);
    int l = 1,r = 0;
    for(int i=1;i<=min(n,20);i++) r+=a[i];
    while(l+1 < r)
    {
        int mid = (l + r) / 2;
        if(ok(mid))
        {
            r = mid;
        }
        else
        {
            l = mid;
        }
    }
    if(ok(l)) printf("%d\n",l);
    else printf("%d\n",r);
    return 0;
}