94. 二叉树的中序遍历

image.png

tag: 栈, 树, 哈希表

    // 递归
    public List<Integer> inorderTraversal1(TreeNode root) {
        List<Integer> list = new LinkedList<>();
        helper(root, list);
        return list;
    }

    private void helper(TreeNode node, List<Integer> list) {
        if (node == null) {
            return;
        }
        if (node.left != null) {
            helper(node.left, list);
        }
        list.add(node.val);
        if (node.right != null) {
            helper(node.right, list);
        }
    }

    // 迭代
    public List<Integer> inorderTraversal2(TreeNode root) {
        List<Integer> list = new LinkedList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            list.add(curr.val);
            curr = curr.right;
        }
        return list;
    }