剑指offer - 整数中1出现的次数

public class Solution {    public int NumberOf1Between1AndN_Solution(int n) {        int res=0;        for(int i=1;i<=n;i++)        {              res+=bt(i);        }        return res;    }    public int bt(int n){        int sum=0;        while(n!=0)        {                   if(n%10==1)                sum++;            n=n/10;        }        return sum;    }}