source
Description
Petya 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 Petya came across a positive integer n. Help him to find the least super lucky number which is not less than n.
Input
The only line contains a positive integer n (1 ≤ n ≤ 109). This number doesn't have leading zeroes.
Output
Output the least super lucky number that is more than or equal to n.
Please, do not use the %lld specificator to read or write 64-bit integers in C++. It is preferred to use the cin, cout streams or the %I64d specificator.
Input
4500
Output
4747
Input
47
Output
47
题意：super lucky number是只含有4，7且4的位数等于7的位数的数字；寻找大于等于n的super lucky number
题解：容易知道，因为4，7的位数相等，幸运数一定是偶数的。所以当num的位数n为奇数的时候，幸运数是位数为n+1位，左边（n+1）/2位为4，右边（n+1）/2位为7，可以保证为符合题意最小的幸运数。num的位数为偶数时，构造左边n/2位为4，右边n/2位为7的幸运数，然后通过next_permutation找到第一个最小的幸运数。
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
int main()
{
char num[50],cpy[50];
scanf("%s",num);
int len=strlen(num);
bool unreal=true;
if(len&1)
{
for(int i=0;i<=len>>1;i++)
{
printf("4");
}
for(int i=0;i<=len>>1;i++)
{
printf("7");
}
}
else
{
for(int i=0;i<len>>1;i++)
{
cpy[i]='4';
}
for(int i=len>>1;i<len;i++)
{
cpy[i]='7';
}
cpy[len]='\0';
do
{
if(strcmp(cpy,num)>=0)
{
printf("%s\n",cpy);
unreal=false;
break;
}
}
while(next_permutation(cpy,cpy+len));
if(unreal)
{
for(int i=0;i<=len>>1;i++)
{
printf("4");
}
for(int i=0;i<=len>>1;i++)
{
printf("7");
}
}
}
}
