344. Reverse String

Write a function that reverses a string. The input string is given as an array of characters char[].

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

You may assume all the characters consist of printable ascii characters.

Example 1:

Input: ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]

Example 2:

Input: ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]

// Iteration
void reverseString(vector<char>& s) { // time: O(n); space: O(1)
    int left = 0, right = s.size() - 1;
    while (left < right) {
        char t = s[left];
        s[left++] = s[right];
        s[right--] = t;
    }
}

void helper(vector<char>& s, int start, int end) {
    if (start >= end) return;
    swap(s[start], s[end]);
    helper(s, start + 1, end - 1);
}
void reverseString(vector<char>& s) {
    helper(s, 0, s.size() - 1);
}