if (word.indexOf(prefix) != 0)

int[] alphabet = new int[26];
for (int i = 0; i < s.length(); i++) alphabet[s.charAt(i) - 'a']++;

digits.charAt(index) - '0'

int num = 0;
for (char c: s.toCharArray()) {
    num = 10 * num + (c - '0');
}

public void swap(char[] s, int left, int right) {
    char temp = s[left];
    s[left] = s[right];
    s[right] = temp; 
}

public void reverse(char[] s, int start, int end) {
    while (start < end) {
        char temp = s[start];
        s[start] = s[end];
        s[end] = temp;
        start++;
        end--;
    }
}