CF 9C水题

输入一个n，求小于n只有1和0组成的数的个数

#include<cstdio>#include<cmath>#include<cstring>#include<iostream>#include<algorithm>using namespace std;#define INF 0x3f3f3f3ftypedef long long ll;const double eps=1e-8;int sgn(double a){return a<-eps?-1:a<eps?0:1;}int main(){    char s[11];scanf("%s",s);    int len=strlen(s);    int ans=0;    for(int i=0;i<len;i++)    {        if(s[i]>'1')        {            ans+=(1<<(len-i))-1;            break;        }        if(s[i]=='1')            ans+=(1<<(len-i-1));    }    printf("%d",ans);}