LeetCode145 二叉树后序遍历

递归：

List<Integer> res = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
        if(root == null) return res;
        if(root.left != null) postorderTraversal(root.left);
        if(root.right != null) postorderTraversal(root.right);
        res.add(root.val);
        return res;
    }

迭代：

public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        if(root == null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while(!stack.isEmpty()){
            TreeNode cur = stack.pop();
            res.add(0,cur.val);
            if(cur.left != null) stack.push(cur.left);
            if(cur.right != null) stack.push(cur.right);
        }
        return res;
    }