Binary Tree Postorder Traversal

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

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [3,2,1].

这道题要求用非递归的方式，这里使用了stack来保存需要访问的节点，用hashmap记录已经添加到result中的节点

import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.List;
import java.util.Stack;

public class Solution {
    private List<Integer> result = new ArrayList<Integer>();
    private HashMap<TreeNode , Integer> visited = new HashMap<TreeNode, Integer>();
    
    public List<Integer> postorderTraversal(TreeNode root) {
        if(null == root)
            return result;
        Stack<TreeNode> stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.isEmpty()){
            root = stack.peek();                                                    //获取栈顶元素的值，但不出栈
            if(root.left != null && visited.get(root.left) == null)
                while(root.left != null && visited.get(root.left) == null){                //寻找左子树,且没有被访问过
                    root = root.left;
                    stack.push(root);
                }//while
            else if(root.right != null && visited.get(root.right) == null)
            {
                root = root.right;
                stack.push(root);
            }
            
            else{
                result.add(root.val);
                visited.put(root, 1);
                stack.pop();                            //出栈
            }            
        }
        
        return result;
    }     
}

递归实现

public class Solution {
    private List<Integer> result = new ArrayList<Integer>();
    
    public List<Integer> postorderTraversal(TreeNode root) {
        postOrder(root);        
        
        return result;
    }
    private void postOrder(TreeNode root){
        if(root != null){
            postorderTraversal(root.left);
            postorderTraversal(root.right);
            result.add(root.val);
        }//if
    }    
}