https://leetcode.com/problems/reverse-string/#/description

array.java

public class Solution {
    public String reverseString(String s) {
        char[] word = s.toCharArray();
        for(int i=0, j=s.length()-1; i<j; i++, j--){
            char temp = word[i];
            word[i] = word[j];
            word[j] = temp;
        }
        return new String(word);
    }
}

StringBuilder.java

public class Solution {
    public String reverseString(String s) {
        StringBuilder sb = new StringBuilder(s);
        for(int i=0, j=s.length()-1; i<j; i++, j--){
            char temp = sb.charAt(i);
            sb.setCharAt(i, sb.charAt(j));
            sb.setCharAt(j, temp);
        }
        return sb.toString();
    }
}

array solution is faster 2ms, stringbuilder slower 6ms, DONOT use insert and add, otherwise it is O(n^2)