66. 二叉树的前序遍历.md

描述

给出一棵二叉树，返回其节点值的前序遍历。

您在真实的面试中是否遇到过这个题？

样例

给出一棵二叉树 {1,#,2,3},

   1
    \
     2
    /
   3

返回 [1,2,3].

Binary Tree Preorder Traversal

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

/**
* Definition of TreeNode:
* public class TreeNode {
*     public int val;
*     public TreeNode left, right;
*     public TreeNode(int val) {
*         this.val = val;
*         this.left = this.right = null;
*     }
* }
*/

public class Solution {
   /**
    * @param root: A Tree
    * @return: Preorder in ArrayList which contains node values.
    */
   public List<Integer> preorderTraversal(TreeNode root) {
       // write your code here

       List<Integer> res = new ArrayList<>();
       Stack<TreeNode> lisk = new Stack<>();

       if(root == null){
           return res;
       }
       lisk.push(root);

       while(!lisk.isEmpty()){

           //取出后加入的：即左孩子
           TreeNode temp = lisk.pop();
           res.add(temp.val);

           //先加入右边的
           if(temp.right != null){
               lisk.push(temp.right);
           }
           //后加入左边的
           if(temp.left != null){
               lisk.push(temp.left);
           }

       }
       return res;

   }
}