p145

 

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