LeetCode-面试题36-二叉搜索树与双向链表

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

    public 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);
    }
}