leetcode-面试题36-二叉搜索树与双向链表

题目描述：

 

 方法：中序遍历 O(N) O(N)

/*
// 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 pre,head;
    public Node treeToDoublyList(Node root) {
        if(root == null) return null;
        dfs(root);
        head.left = pre;
        pre.right = head;
        return head;
    }
    void dfs(Node cur){
        if(cur==null) return;
        dfs(cur.left);
        if(pre!=null) pre.right =cur;
        else head = cur;
        cur.left = pre;
        pre = cur;
        dfs(cur.right);
    }
}