ztr loves lucky numbers
Time Limit: 2000/1000 MS (Java/Others) Memory Limit: 65536/65536 K (Java/Others)
Problem Description
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
(1≤n≤105) cases
For each cases:
The only line contains a positive integer
n(1≤n≤1018). This number doesn't have leading zeroes.
Output
For each cases
Output the answer
Sample Input
2 4500 47
Sample Output
4747 47
解题思路：其实是一道比较简单的题目，用dfs搜一发即可。自己的代码写的好搓，参考了别人的代码。
#include<cstdio>
#include<cstring>
using namespace std;
char s[50];
bool found;
int four,seven,len;
void write(int len)
{
for(int i = 0; i < len/2; i++) printf("4");
for(int i = 0; i < len/2; i++) printf("7");
printf("\n");
}
void dfs(long long ans, long long res, int lens)
{
if(lens == len && !found){
found = true;
printf("%I64d\n", ans);
}
if(found) return;
res = res * 10 + s[lens] - '0';
if(res <= ans * 10 + 4 && four < len / 2){
four++;
dfs(ans * 10 + 4, res, lens + 1);
four--;
res /= 10;
}
if(found) return;
if(res <= ans * 10 + 7 && seven < len / 2){
seven++;
dfs(ans * 10 + 7, res, lens + 1);
seven--;
res /= 10;
}
}
int main()
{
int T;scanf("%d", &T);
while(T--){
scanf("%s", s);
len = strlen(s);
if(len & 1) write(len + 1);
else{
four = seven = 0;
found = false;
dfs(0, 0, 0);
if(!found) write(len + 2);
}
}
return 0;
}
