字典树

class Solution {
    public int findKthNumber(int n, int k) {
    int ans = 1;
    while(k>1){
        int count = dfs(ans,n);
        //求出子树的数量和K比较，若大于k则在子树中找，否则在循环下一个节点
        if(count<k){
            ans++;
            k-=count;
        }else{
            ans*=10;
            k--;
        }
    }
    return ans;
}
    //计算子树中的总数
    public int dfs(long ans,long n){
        int tem = 0;
        long left = n;
        long right = n;
        while(left<=right){
            tem += Math.min(right,n) - left +1;
            left*=10;
            right=right*10+9;
        }
        return tem;
    }
}