小米oj #40 找小“3”

把各个数位上的"3"的出现次数找找一边就行了，不会数位DP，但水过去了

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
int main(){
    ll n;
    while(~scanf("%lld",&n)){
        ll ans=0,t1=1,k=0,no;
        while(n){
            ll t=t1;
            if(t1==1)t*=2;
            ans+=n/10*t/2;
            no=n%10;
            if(no>3)ans+=t/2;
            else if(no==3)ans+=(t1==1?1:(k+1)/2);
            k+=no*t1;
            t1*=10;
            n/=10;
        }
        printf("%lld\n",ans);
    }
    return 0;
}