Leetcode_剑指offer36_二叉搜索树与双向链表_树

中序遍历 左根右，递归

class Solution {
    Node head, last;

    public Node treeToDoublyList(Node root) {
        if (root == null) {
            return null;
        }
        dfs(root);

        head.left = last;
        last.right = head;
        return head;
    }

    void dfs(Node node) {
        if (node == null) {
            return;
        }
        dfs(node.left);
        if (last != null) {
            last.right = node;
        } else {
            head = node;
        }
        node.left = last;
        last = node;
        dfs(node.right);
    }
}