新生训练第二期白银组题一CodeForces-110A【水题】

题目网址：https://vjudge.net/problem/CodeForces-110A
Petya loves lucky numbers. We all know that lucky numbers are the positive integers whose decimal representations contain only the lucky digits 4 and 7. For example, numbers 47, 744, 4 are lucky and 5, 17, 467 are not.

Unfortunately, not all numbers are lucky. Petya calls a number nearly lucky if the number of lucky digits in it is a lucky number. He wonders whether number n is a nearly lucky number.

Input
The only line contains an integer n (1 ≤ n ≤ 1018).

Please do not use the %lld specificator to read or write 64-bit numbers in С++. It is preferred to use the cin, cout streams or the %I64d specificator.

Output
Print on the single line “YES” if n is a nearly lucky number. Otherwise, print “NO” (without the quotes).

Examples
Input
40047
Output
NO
Input
7747774
Output
YES
Input
1000000000000000000
Output
NO
Note
In the first sample there are 3 lucky digits (first one and last two), so the answer is “NO”.

In the second sample there are 7 lucky digits, 7 is lucky number, so the answer is “YES”.

In the third sample there are no lucky digits, so the answer is “NO”.

理解：题目大概意思是只有4或7组成的数字称为幸运数，然后定义一个新的“幸运数”，这个“幸运数”是指一串数字中4或7出现的总数是否也为幸运数。
思路：定义一个字符数组来输入，再读取，然后累加4或7的个数，最后再判断累加数是否为幸运数。

#include
#include
using namespace std;
int main()
{	
	char n[1000];	//题目要求n可达到10的18次方，而long long 的范围任然没那么大所以采用字符数组更好
	int num=0,flag=0,len;
	cin>>n;
	len=strlen(n);
	for(int i=0;i

小结：水题。。。范围处需注意下。。。（所以4和7哪里幸运了/摊手）