Leetcode144:二叉树的前序遍历

给定一个二叉树，返回它的 前序 遍历。

示例:
输入: [1,null,2,3]
1
\
2
/
3
输出: [1,2,3]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

	//解法1 递归法
	public List<Integer> preorderTraversal(TreeNode root) {

		List<Integer> resultList = new ArrayList<>();
		return preorderTraversal(root,resultList);
	}

	private List<Integer> preorderTraversal(TreeNode root, List<Integer> resultList) {

		if (root==null){
			return resultList;
		}
		resultList.add(root.val);
		preorderTraversal(root.left, resultList);
		preorderTraversal(root.right, resultList);
		return resultList;
	}

	//解法2 迭代法
	public List<Integer> preorderTraversal2(TreeNode root){
		Stack<State> stack = new Stack<>();
		stack.push(new State("go", root));
		List<Integer> resultList = new ArrayList<>();

		while (!stack.empty()) {
			State state = stack.pop();
			if (state.action.equals("go") && state.target!=null) {
				stack.push(new State("go", state.target.right));
				stack.push(new State("go", state.target.left));
				stack.push(new State("add", state.target));
			} else if (state.action.equals("add") && state.target != null) {
				resultList.add(state.target.val);
			}
		}

		return resultList;
	}

	private class State{
		String action;
		TreeNode target;

		public State(String action, TreeNode target) {
			this.action = action;
			this.target = target;
		}
	}