344 Reverse String

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