HDU 5676 ztr loves lucky numbers

ztr loves lucky numbers. Everybody knows that positive integers are lucky if their decimal representation doesn’t contain digits other than 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Lucky number is super lucky if it’s decimal representation contains equal amount of digits 4 and 7. For example, numbers 47, 7744, 474477 are super lucky and 4, 744, 467 are not.

One day ztr came across a positive integer n. Help him to find the least super lucky number which is not less than n.

Input

There are T (1≤T≤10^5) cases

For each cases:

The only line contains a positive integer n(1≤n≤10^18) . This number doesn’t have leading zeroes.

Output

For each cases

Output the answer

Sample Input

2

4500

47

Sample Output

4747

47

思路：

可以用STL的next_permutation。不过我写的时候不会这个（尴尬），所以还是自己老老实实写吧。

先递归build一个记录了1到10^18的所有super lucky numbers的数组，之后只需要每次遍历就行了，

可以再加上二分法不过这里我没用也过了，数据不是很严，主要考的是这种类似桶排序的思路。

代码：

#include <cstdio>
#include <cstring>
#include <iostream>

using namespace std;

long long board[100000];
int flag;

void build(int a,int b,long long t){
if(a == 0 && b == 0){
board[flag++] = t;
return ;
}
if(a!=0)build(a-1,b,t*10+4);
if(b!=0)build(a,b-1,t*10+7);
return;
}

int main(){
int T;
cin>>T;
for(int i=1 ; i<=9 ; i++){
build(i,i,0);
}
long long N;
while(T--){
scanf("%lld",&N);
int i;
for(i=0 ; i<flag ; i++){
if(board[i]>=N){
printf("%lld\n",board[i]);
break;
}
}
if(i == flag)printf("44444444447777777777\n");
}
return 0;
}