Nearly Lucky Number
题目：
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”.(个数要为4||7个)
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”.
问题地址
#include<iostream>
using namespace std;
int main()
{
char n[20];
while (cin>>n)
{
int j = 0, i = 0;
for (int i = 0;n[i]!=' '; i++)
{
if (n[i] == '7' || n[i] == '4')
{
j++;
}
}
if (j==i-1)
{
cout << "YES";
}
else
{
if (j==4||j==7)
{
cout << "YES";
}
else
{
cout << "NO";
}
}
}
return 0;
}
思考：使用字符数组，直接输入，简单的条件判断（多练才能上手）。
不用scanf，慢，超时。