LeetCode 917. Reverse Only Letters (javascript)

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:

1. S.length <= 100
2. 33 <= S[i].ASCIIcode <= 122 
3. S doesn’t contain \ or "

Idea:

Two pointers

Solution:

/**
 * @param {string} S
 * @return {string}
 */
var reverseOnlyLetters = function(S) {
    let l = 0;
    let r = S.length - 1;
    let arr = S.split('');
    
    while (l < r) {
        if (!isAlpha(arr[l])) l++;
        if (!isAlpha(arr[r])) r--;
        if (isAlpha(arr[l]) && isAlpha(arr[r])) {
            // swap
            [arr[l], arr[r]] = [arr[r], arr[l]];
            l++
            r--;
        }
    }
    return arr.join('');
};

var isAlpha = (c) => /[a-zA-Z]/.test(c);