是个人都会做。
我能想到三种方法：
// 1. two pointers, i < j
// 2. j >= 0 ; j --
// 3. Stack

Here I implement the 3rd one:

    public String reverse(String s) {
        Stack<Character> stack = new Stack<>();
        for (int i = 0; i < s.length(); i++) {
            stack.push(s.charAt(i));
        }
        StringBuilder sb = new StringBuilder();
        while (!stack.isEmpty()) {
            sb.append(stack.pop());
        }
        return sb.toString();
    }

有人实现了6种：
https://discuss.leetcode.com/topic/43296/java-simple-and-clean-with-explanations-6-solutions