解题思路：
https://leetcode-cn.com/problems/k-th-smallest-in-lexicographical-order/solution/dfsmo-ni-jian-zhi-guo-cheng-by-muyids/

typedef long long LL;
class Solution {
public:
    int findKthNumber(int n, int k) {
        int prefix = 1;
        k--;
        while(k>0){
            int cnt = getCnt(n,prefix);
            if(cnt<=k){
                k-=cnt;
                prefix++;
            }else{
                k--;
                prefix*=10;
            }
        }
        return prefix;
    }

    int getCnt(LL n,LL prefix){
        LL cnt = 0;
        for(LL first=prefix,second=prefix+1;first<=n;first*=10,second*=10){
            cnt += min(n+1,second) - first;
        }
        return cnt;
    }
};