145.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].

/**
 * 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> left;
        List<Integer> right;
        List<Integer> list = new LinkedList<Integer>();
        
        if(root == null)
            return list;
        left = postorderTraversal(root.left);
        right = postorderTraversal(root.right);
        for(int i = 0; i < left.size(); i++)
            list.add(left.get(i));
        for(int j = 0; j < right.size(); j++)
            list.add(right.get(j));
            
        list.add(root.val);
        return list;
    }
}