【二分】无限序列

描述

1101001000100001000001.......

求第Ai位的数是什么

Ai<=2000000

因为可以推出所有1位置的序号

就可以用一个数组给存起来

又因序号是递增的

所以就可以二分来查找

 1 # include<cstdio>

 2 # include<cstring>

 3 # include<iostream>

 4 # include<algorithm>

 5 using namespace std;

 6 typedef unsigned long long LL;

 7 const int maxn=50000;

 8 const int INF=900000000;

 9 LL f[maxn];

10 int two_find(LL l,LL r,LL x){

11     while(l<r){

12         LL mid=(l+r)>>1;

13         if(f[mid]==x)return 1;

14         else if(f[mid]>x)r=mid;

15         else l=mid+1;

16     }

17     return 0;

18 }

19 int main(){

20     ios::sync_with_stdio(false);

21     LL cur=0,tb=1,num=1,n,x;

22     while(f[cur++]<=INF){

23         f[cur]=num;

24         num=num+tb;

25         tb++;

26     }

27     cin>>n;

28     for(int i=1;i<=n;i++){

29         cin>>x;

30         if(two_find(1,cur,x))printf("1\n");

31         else printf("0\n");

32     }

33     return 0;

34 }