145.Binary Tree Postorder Traversal （二叉树后序遍历）

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].

代码：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        if(root!=null){ 
             res.addAll(postorderTraversal(root.left));           
             res.addAll(postorderTraversal(root.right));  
             res.add(root.val);
         }
        return res;
    }
}