C++ 解法, 执行用时: 50ms, 内存消耗: 504K, 提交时间: 2021-10-02 16:34:02

using namespace std;
#include <bits/stdc++.h>
#define ll long long
ll pw[19];
ll x,k;
ll print(ll v,int flag){
	int w=log10(v);
	ll u=v;
	for (int i=flag;i<=w;++i) u=u*10+v/pw[i]%10;
	return u;
}
int main(){
//	freopen("in.txt","r",stdin);
	pw[0]=1;
	for (int i=1;i<=18;++i)
		pw[i]=pw[i-1]*10;
	scanf("%lld%lld",&x,&k);
	ll w=log10(x)+1,t=x/pw[w/2],h=(w+1)/2;
	if (print(t,w&1)>=x)
		--k;
	while (k>0){
		--k;
		++t;
		if (t==pw[h]){
			++w;
			if (w&1^1)
				t=pw[h-1];
			else	
				++h;
		}
	}
	ll ans=print(t,w&1);
	printf("%lld\n",ans);
	return 0;
}