分类讨论: 讨论每位上1出现的次数

// 1~n中  1出现的次数

class Solution {
public:
    int numberOf1Between1AndN_Solution(int n) {
        if(!n) return 0;
        vector<int> number, prefix;
        while(n) number.push_back(n%10),prefix.push_back(n),n/=10;

        int left=0;
        int res=0;

        for(int i=number.size()-1;i>=0;i--) {
            int t=0, right=0;
            t=pow(10, i);
            right=prefix[0]-prefix[i]*t;   
            res += left*t;
            if(number[i]==1) res += right+1;
            else if(number[i]>1) res +=t;
            left = left*10 + number[i];
        }
        return res;
    }
};