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**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].

 

牛解法:postorder就是L-R-ROOT,这里先ROOT-R-L,再把结果reverse

public class Solution {public List<Integer> postorderTraversal(TreeNode root) {    List<Integer> results = new ArrayList<Integer>();    Deque<TreeNode> stack = new ArrayDeque<TreeNode>();    while (!stack.isEmpty() || root != null) {        if (root != null) {            stack.push(root);            results.add(root.val);            root = root.right;        } else {            root = stack.pop().left;        }    }    Collections.reverse(results);    return results;}}

关于deque:https://docs.oracle.com/javase/7/docs/api/java/util/Deque.html

reference:https://leetcode.com/discuss/9736/accepted-code-with-explaination-does-anyone-have-better-idea

**Binary Tree Postorder Traversal