[leetcode] Binary Tree Postorder Traversal

题目：（Tree ，Stack）

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].

题解：

binary tree 三部曲里面最难的一个。多看看。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ArrayList<Integer> postorderTraversal(TreeNode root) {
       ArrayList<Integer> result = new ArrayList<Integer>();
       if(root==null)
            return result;
       
       Stack<TreeNode> stack=new Stack<TreeNode>();
       stack.push(root);
       TreeNode prev=null;
       
       while(!stack.isEmpty()){
           TreeNode current=stack.peek();
           if(prev==null||prev.left==current||prev.right==current)
           {
               if(current.left!=null)
                   stack.push(current.left);
               else if(current.right!=null)
                   stack.push(current.right);
               else
               {
                   stack.pop();
                   result.add(current.val);
               }
           }else if(prev==current.left)
           {
               if(current.right!=null)
                   stack.push(current.right); 
               else
               {
                   stack.pop();
                   result.add(current.val);
               }
           }else
           {
               stack.pop();
               result.add(current.val);
           }
           prev=current;
       }
       return result;
    }
}