[leetcode]非递归先序遍历二叉树(Binary Tree Preorder Traversal)

题目描述是这样的：

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

要求使用非递归方法。

可以利用栈来实现。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList
  
    preorderTraversal(TreeNode root) {
       ArrayList
   
     list=new ArrayList
    
     (); Stack
     
       stack=new Stack
      
       (); if(root!=null){ stack.push(root); while(!stack.empty()){ list.add(stack.peek().val);//这里只能用peek()，因为如果pop()的话会在获得val值的同时将结点弹出栈。 root=stack.pop(); if(root.right!=null){ stack.push(root.right);//右孩子先入栈，后出栈 } if(root.left!=null){ stack.push(root.left);//左孩子后入栈，先出栈 } } } return list; } }