[LeetCode] 917. Reverse Only Letters

Problem

Given a string S, return the "reversed" string where all characters that are not a letter stay in the same place, and all letters reverse their positions.

Example 1:

Input: "ab-cd"
Output: "dc-ba"
Example 2:

Input: "a-bC-dEf-ghIj"
Output: "j-Ih-gfE-dCba"
Example 3:

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

Note:

S.length <= 100
33 <= S[i].ASCIIcode <= 122 
S doesn't contain \ or "

Solution

class Solution {
    public String reverseOnlyLetters(String S) {
        char[] str = S.toCharArray();
        int i = 0, j = str.length-1;
        while (i < j) {
            while (i < j && !Character.isLetter(str[i])) i++;
            while (i < j && !Character.isLetter(str[j])) j--;
            swap(str, i++, j--);
        }
        StringBuilder sb = new StringBuilder();
        sb.append(str);
        return sb.toString();
    }
    private void swap(char[] str, int i, int j) {
        char temp = str[i];
        str[i] = str[j];
        str[j] = temp;
    }
}