原题链接: https://oj.leetcode.com/problems/binary-tree-postorder-traversal/

public List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();

    if (root == null) return result;

    LinkedList<TreeNode> stack = new LinkedList<TreeNode>();
    TreeNode prev = null;

    while (root != null || !stack.isEmpty()) {
        if (root != null) {
            stack.push(root);
            root = root.left;
        } else {
            TreeNode peek = stack.peek();

            if (peek.right != null && peek.right != prev) {
                root = peek.right;
            } else {
                stack.pop();
                result.add(peek.val);
                prev = peek;
            }
        }
    }

    return result;
}

public List<Integer> postorderTraversal(TreeNode root) {
    List<Integer> result = new ArrayList<Integer>();

    if (root == null) return result;

    result.addAll(postorderTraversal(root.left));
    result.addAll(postorderTraversal(root.right));
    result.add(root.val);

    return result;
}