LeetCode 917 Reverse Only Letters

Input: "ab-cd"
Output: "dc-ba"

Input: "a-bC-dEf-ghIj"
Output: "j-Ih-gfE-dCba"

Input: "Test1ng-Leet=code-Q!"
Output: "Qedo1ct-eeLg=ntse-T!"

class Solution {
    public String reverseOnlyLetters(String S) {
        char[] chars = S.toCharArray();
        int i = 0, j = chars.length - 1;
        while (i < j) {
            while (i < j && !isLetter(chars[i])) {
                i++;
            }
            while (i < j && !isLetter(chars[j])) {
                j--;
            }
            if (i < j) {
                char temp = chars[i];
                chars[i] = chars[j];
                chars[j] = temp;
                i++;
                j--;
            }
        }
        return new String(chars);
    }

    private boolean isLetter(char c) {
        return (c >= 65 && c <= 90) || (c >= 97 && c <= 122);
    }
}