http://acm.hdu.edu.cn/contests/contest_showproblem.php?pid=1008&cid=843

题解：模板题

/*
*@Author:   STZG
*@Language: C++
*/
#include <bits/stdc++.h>
#include<iostream>
#include<algorithm>
#include<cstdlib>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<bitset>
#include<queue>
#include<deque>
#include<stack>
#include<cmath>
#include<list>
#include<map>
#include<set>
//#define DEBUG
#define RI register int
using namespace std;
typedef long long ll;
//typedef __int128 lll;
const int N=100000+10;
const int M=100000+10;
const int MOD=1e9+7;
const double PI = acos(-1.0);
const double EXP = 1E-8;
const int INF = 0x3f3f3f3f;
int t,n,m,k,q;
int ans,cnt,flag,temp,sum;
int a[N];
char str;
struct node{};
	int findKthNumber(int n, int k) {
	    //*************************
	    //第一部分，得到树群的最大深度。
		if (n<10)
			return k;
		int sum = 9;
		int base = 1;
		int deep = 0;
		while (n>sum) {
			base *= 10;
			++deep;
			sum += 9 * base;
		}
		//*************************
		//第二部分，确定k在左侧树群还是在右侧树群。
		int redund = n - (base - 1);
		int lisNum = redund;
		while (base != 1) {
			n /= 10;
			base /= 10;
			int tmp = n - (base - 1);
			//cout << tmp << endl;
			lisNum += tmp;
		}
		if (k > lisNum) {
			--deep;
			k -= redund;
		}
		//*************************
		//第三部分，在k所归属的树中找到其值。
		//cout<<" +"<<endl;
		int partSum = 0;
		vector<int>work(deep + 1, 0);
		for (int i = 0; i <= deep; ++i) {
			partSum += pow(10, i);
			work[deep - i] = partSum;
		}
		int seq = (k - 1) / partSum + 1;
		int rem = (k - 1) % partSum;
		string result;
		result.push_back('0' + seq);
		for (int i = 1; i <= deep && rem != 0; ++i) {
			--rem;
			int order = rem / work[i];
			rem = rem % work[i];
			result.push_back('0' + order);
		}
		return stoi(result);
	}

int main()
{
#ifdef DEBUG
	freopen("input.in", "r", stdin);
	//freopen("output.out", "w", stdout);
#endif
    //ios::sync_with_stdio(false);
    //cin.tie(0);
    //cout.tie(0);
    scanf("%d",&t);
    while(t--){
        scanf("%d%d",&n,&k);
        printf("%d\n",findKthNumber(n,k));
    }

#ifdef DEBUG
	printf("Time cost : %lf s\n",(double)clock()/CLOCKS_PER_SEC);
#endif
    //cout << "Hello world!" << endl;
    return 0;
}