7의 개수

class Solution {
    public int solution(int[] array) {
        int answer = 0;

        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < array.length; i++) {
            sb.append(array[i]);
        }

        for (int i = 0; i < sb.length(); i++) {
            int c = sb.charAt(i) - '0';
            if (c == 7) {
                answer++;
            }
        }
        return answer;
    }
}