C++(clang++11) 解法, 执行用时: 3ms, 内存消耗: 380K, 提交时间: 2020-12-26 19:55:18

#include<iostream>
using namespace std;
long long n,ans,pw[19];
int s;
int main(){
	cin>>n;
	for(long long i=10;i<=n;i*=10)pw[++s]=i;
	++s;
	if(s&1)cout<<pw[s>>1]-1<<endl;
	else
		cout<<(n/pw[s>>1]<=n%pw[s>>1]?n/pw[s>>1]:n/pw[s>>1]-1)<<endl;
	return 0;
} 

pypy3(pypy3.6.1) 解法, 执行用时: 51ms, 内存消耗: 25536K, 提交时间: 2021-03-12 22:18:09

n = int(input())
l = 1
r = 10 ** 13
while r - l > 1:
	mid = (l + r) // 2
	s = str(mid)
	s = s + s
	if int(s) <= n:
		l = mid
	else:
		r = mid
print(l)

Python3(3.9) 解法, 执行用时: 20ms, 内存消耗: 2816K, 提交时间: 2020-12-26 19:03:57

n = int(input())
l = 1
r = 10 ** 12
while r - l > 1:
	mid = (l + r) // 2
	s = str(mid)
	s = s + s
	if int(s) <= n:
		l = mid
	else:
		r = mid
print(l)