剑指 Offer 36. 二叉搜索树与双向链表

class Solution {
    Node pre, head;

    public Node treeToDoublyList(Node root) {
        if (root == null) return null;
        dfs(root);
        head.left = pre;
        pre.right = head;
        return head;
    }

    private void dfs(Node root) {
        if (root == null) return;
        dfs(root.left);
        if (pre == null) {
            head = root;
        } else {
            pre.right = root;
        }
        root.left = pre;
        pre = root;
        dfs(root.right);
    }
}