二叉搜尋樹的先序遍歷序列是遞增序列
設定pre指標來記錄上一次訪問結果

 1 /*
 2 // Definition for a Node.
 3 class Node {
 4 public:
 5     int val;
 6     Node* left;
 7     Node* right;
 8 
 9     Node() {}
10 
11     Node(int _val) {
12         val = _val;
13         left = NULL;
14         right = NULL;
15     }
16 
17     Node(int _val, Node* _left, Node* _right) {
 
18         val = _val;
19         left = _left;
20         right = _right;
21     }
22 };
23 */
24 class Solution {
25 private:
26     Node * pre ;
27     Node * head;
28     Node * last ;
29 
30 public:
31     Node* treeToDoublyList(Node* root) {
32         if(root ==NULL)
33             return NULL;
 
34         pre = NULL;
35         head = NULL;
36         last =NULL;
37         visit(root);
38         head->left = pre;
39         pre->right = head;
40         return head;
41 
42         
43     }
44     void visit(Node* root)
45     {
46         if(root == NULL)
47             return;
48         visit(root->left);
 
49         if(head == NULL)
50         {    
51             head = root;
52             pre = root;
53         }
54         else
55         {
56             pre->right = root;
57             root->left = pre;
58             pre = root;
59         }
60         visit(root->right);
61     }
62 };