二叉树后序遍历

二叉树后序遍历，分为递归方法和非递归方法：

递归：

private static List<Integer> postList = new ArrayList<>();
public static List<Integer> postorderTraversalRec(TreeNode root){
    if (null == root){
        return postList;
    }
    postorder(root);
    return postList;
}
private static void postorder(TreeNode root){
    if (null == root){
        return;
    }
    postorder(root.left);
    postorder(root.right);
    postList.add(root.val);
}

非递归：

public static List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> list = new ArrayList<>();
    if (null == root){
        return list;
    }
    Stack<TreeNode> stack = new Stack<>();
    stack.push(root);
    while (!stack.isEmpty()){
        TreeNode node = stack.pop();
        list.add(node.val);
        if (null != node.left){
            stack.push(node.left);
        }
        if (null != node.right){
            stack.push(node.right);
        }
    }
    Collections.reverse(list);
    return list;
}