917. Reverse Only Letters

https://leetcode.com/problems/reverse-only-letters/

1. Two pointers:

//0 ms    6.2 MB
class Solution {
public:
    string reverseOnlyLetters(string S) {
        int n = S.length();
        int l = 0;
        int r = n - 1;
        while(l < r){
            if(isalpha(S[l]) && isalpha(S[r])){ //if S[l] and S[r] are both letters
                char tmp = S[l];
                S[l] = S[r];
                S[r] = tmp;
                l++;
                r--;
            } else if(!isalpha(S[l]) && isalpha(S[r])){
                l++;
            } else if(isalpha(S[l]) && !isalpha(S[r])){
                r--;
            } else if(!isalpha(S[l]) && !isalpha(S[r])){
                l++;
                r--;
            }
        }
        return S;
    }
};

2. Two pointers(optimized):

//0 ms    6.2 MB
class Solution {
public:
    string reverseOnlyLetters(string S) {  
        int l = 0;
        int r = S.length()-1;
        while(l<r){
            if(isalpha(S[l]) && isalpha(S[r])){
                swap(S[l], S[r]);
                l++;
                r--;
            } else {
                if(!isalpha(S[l]))
                    l++;
                if(!isalpha(S[r]))
                    r--;
            }
        }
        return S;
    }
};