A - Leading 1s
题意：设 \(f(x)\) 为 \(x\) 这个数的前导 1 的个数。给定一个数 \(n (1<=n<=10^{15})\)
code:

#include<bits/stdc++.h>
#define ll long long
using namespace std;
int main()
{
	ll n;
	cin >> n;
	ll ans = 0;
	for(int x = 1; x <= 16; ++x)
	{
		ll w = 0;
		for(int j = 0; j < x; ++j)
			w = w * 10 + 1;
		ll p = w;
		for(int y = x; y <= 16; ++y)
		{
			if(w <= n)
			{
				ans += min(n, p) - w + 1;
			}
			w *= 10;
			p = 10 * p + 9;
		}
	}
	cout << ans << endl;
	return 0;
}