【Leetcode】Binary Tree Preorder Traversal

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

题目：

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].

Note: Recursive solution is trivial, could you do it iteratively?

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leftstack.push(tmp.right);if (tmp.left != null)stack.push(tmp.left);}return list;}