Codeforces 문제 푸는 길 - 110A Nearly Lucky Number

A. Nearly Lucky Number
time limit per test
2 seconds
memory limit per test
256 megabytes
input
standard input
output
standard output
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".
제목 대의: 본 문제는 lucky number가 아니라 nearly lucky number로 판단됩니다. 이 숫자에는 lucky number 4,7의 개수가 포함되어 있습니다. 이 개수는 lucky number(즉 4 or 7로 구성된 숫자)입니까?
문제풀이 사고방식: 먼저 이 입력 숫자에 lucky number가 포함된 개수를 계산하여 이 개수가 4or 7만 함유되어 있는지 검사하면 된다.
다음은 문제풀이 코드(java 구현)

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner = new Scanner(System.in);
		String string = scanner.nextLine();
		
		int count = 0;
		for(int i = 0;i < string.length();i++){
			if(string.charAt(i) == '4' || string.charAt(i) == '7'){
				count++;
			}
		}
		
		boolean flag = false;
		String string2 = String.valueOf(count);
		for(int i = 0;i < string2.length();i++){
			if(string2.charAt(i) == '4' || string2.charAt(i) == '7'){
				flag = true;
				continue;
			}else{
				flag = false;
				break;
			}
		}
		if(flag){
			System.out.println("YES");
		}else{
			System.out.println("NO");
		}
		scanner.close();
	}
}