LeetCode-Binary Tree Preorder Traversal

LeetCode-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].

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

Source code: Java version
Algorithm Analysis: Stack. Time complexity O (n), space complexity O (n)

/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; } * } */public class Solution {    public List
 
   preorderTraversal(TreeNode root) {        List
  
    result=new ArrayList
   
    ();        Stack
    
      stack=new Stack
     
      ();        TreeNode node=root;        do {            while(node!=null) {                result.add(node.val);                if(node.right!=null) {                    stack.push(node.right);                }                node=node.left;            }            if(!stack.isEmpty()) {                node=stack.pop();            }        }while(!stack.isEmpty() || node!=null);        return result;    }}