atcoder beginner contest 340c -爱代码爱编程

problem link

Naively, a brute force recursion solution be implemented with $\mathcal{O}(n)$ complexity.

int work(int x)
{
	if(x==1)return 0;
	return x+work(x>>1)+work((x>>1)+(x&1))
}

However, since all possible $x$ can be represented as $n\cdot 2^{-k}+[0/1]$, the number of possible $x$ does not exceed $2\cdot {\log }_2(n)$

Then, we can intuitively implement a memorization search with map.

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<map>
using namespace std;
map <long long,long long> f;
long long n,ans;
long long work(long long x)
{
	if(x==1)return 0;
	if(f[x])return f[x];
	return f[x]=x+work(x>>1)+work((x>>1)+(x&1));
}
int main()
{
	cin>>n;
	cout<<work(n)<<endl;
	return 0;
}