36-二叉搜索树与双向链表

输入一棵二叉搜索树，将该二叉搜索树转换成一个排序的循环双向链表。要求不能创建任何新的节点，只能调整树中节点指针的指向。
注意审题，看清是否需要循环链表。

生成双向链表模板：

    Node pre = null;
    Node head = null;
    for (...) {
        Node tmp = ...;
        if (head == null) head = tmp;
        if (pre != null) {
            pre.right = tmp;
        }
        tmp.left = pre;
        pre = tmp;
    }

    // 如果需要循环链表
    head.left = pre;
    pre.right = head;

答案：

/*
// Definition for a Node.
class Node {
    public int val;
    public Node left;
    public Node right;

    public Node() {}

    public Node(int _val) {
        val = _val;
    }

    public Node(int _val,Node _left,Node _right) {
        val = _val;
        left = _left;
        right = _right;
    }
};
*/
class Solution {
    Node head;
    Node pre;
    public Node treeToDoublyList(Node root) {
        if (root == null) return null;
        recur(root);
        head.left = pre;
        pre.right = head;
        return head;
    }

    private void recur(Node node) {
        if (node == null) return;
        recur(node.left);

        if (pre != null) {
            pre.right = node;
        } else {
            head = node;
        }
        node.left = pre;
        pre = node;
        recur(node.right);
    }
}