Java for LeetCode 144 Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

return [1,2,3].

二叉树的前序遍历，根节点→左子树→右子树

解题思路一：

递归实现，JAVA实现如下：

    public List<Integer> preorderTraversal(TreeNode root) {
		List<Integer> list = new ArrayList<Integer>();
		if (root == null)
			return list;
		list.add(root.val);
		list.addAll(preorderTraversal(root.left));
		list.addAll(preorderTraversal(root.right));
		return list;
    }

解题思路二：

使用stack实现，JAVA实现如下：

	public List<Integer> preorderTraversal(TreeNode root) {
		List<Integer> list = new ArrayList<Integer>();
		if (root == null)
			return list;
		Stack<TreeNode> stack = new Stack<TreeNode>();
		stack.push(root);
		TreeNode pop = root;
		while (!stack.isEmpty()) {
			pop = stack.pop();
			list.add(pop.val);
			if (pop.right != null)
				stack.add(pop.right);
			if (pop.left != null)
				stack.add(pop.left);
		}
		return list;
	}