917. Reverse Only Letters

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 "

// Two Pointers
string reverseOnlyLetters(string S) { // time: O(n); space: O(n)
    string res = S;
    for (int i = 0, j = res.length() - 1; i < j; ++i, --j) {
        while (i < j && !isalpha(res[i])) ++i;
        while (i < j && !isalpha(res[j])) --j;
        swap(res[i], res[j]);
    }
    return res;
}