【Leetcode】Binary Tree Preorder Traversal

题目链接：https://leetcode.com/problems/binary-tree-preorder-traversal/

题目：

preorder

For example:

Given binary tree {1,#,2,3},

1 \ 2 / 3

[1,2,3].

Note:

思路：

前序遍历用栈保存，注意要先压入右结点，在压入左结点

算法：

public List<Integer> preorderTraversal(TreeNode root) {
		List<Integer> list = new ArrayList<Integer>();
		Stack<TreeNode> stack = new Stack<TreeNode>();
		if (root == null)
			return list;
		stack.push(root);
		while (!stack.isEmpty()) {
			TreeNode tmp = stack.pop();
			list.add(tmp.val);
			if (tmp.right != null) //先压right，先出left，不断的先访问left
				stack.push(tmp.right);
			if (tmp.left != null)
				stack.push(tmp.left);
		}
		return list;
	}