1576. Replace All ?’s to Avoid Consecutive Repeating Characters #

class Solution {
    private Random random = new Random();
    public Character findCandidate(Character ban1, Character ban2) {
        Character ans = 'a';
        while (ans == ban1 || ans == ban2) {
            ans = (char)('a' + this.random.nextInt(26));
        }
        return ans;
    }
    public String modifyString(String s) {
        char[] charArray = s.toCharArray();
        for (int i = 0; i < charArray.length; i++) {
            if (charArray[i] == '?') {
                if (i == 0) {
                    if (i+1 < charArray.length) charArray[i] = findCandidate(charArray[i+1], null);
                    else charArray[i] = 'a';
                }
                else if (i == charArray.length - 1) charArray[i] = findCandidate(charArray[i-1], null);
                else charArray[i] = findCandidate(charArray[i-1], charArray[i+1]);
            }
        }
        return String.valueOf(charArray);
    }
}