Convert Sorted List to Binary Search Tree

Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST.

思想： 利用递归的思想

注意事项：递归是一个输入是一个链表的头指针，同时要链表的尾部节点为null（要保证），否则排序时会导致错乱

java代码：

1. public TreeNode sortedListToBST(ListNode head) {
2. TreeNode root=null;
3. if(head == null) return root;
4. if(head != null && head.next==null) return new TreeNode(head.val);
5. if(head!=null && head.next!=null && head.next.next==null) {
6. TreeNode left = new TreeNode(head.val);
7. root=new TreeNode(head.next.val);
8. root.left=left;
9. return root;
10. }
11. ListNode slow = head;
12. ListNode fast = head;
13. ListNode pre = slow;
14. while(fast!=null && fast.next!=null) {
15. fast = fast.next.next;
16. pre=slow;
17. slow = slow.next;
18. }
19. fast = slow.next;
20. slow.next=null; 
21. root = new TreeNode(slow.val); //slow指向中间节点
22. pre.next=null;  //第一个链表的节点尾部为null
23. TreeNode left = sortedListToBST(head);
24. TreeNode right = sortedListToBST(fast);
25. root.left=left;
26. root.right=right;
27. return root;
28. }

思想二：

递归：利用每次增加中间节点的方式，并结合节点个数进行递归。

1. TreeNode* addNode(ListNode * &list,int start,int end) {
2. if(start>end) return NULL;
3. int mid=start+(end-start)/2;
4. TreeNode * left = addNode(list,start,mid-1);
5. TreeNode *parent=new TreeNode(list->val);
6. parent->left=left;
7. list=list->next;  // list指向下一个节点
8. parent->right=addNode(list,mid+1,end);
9. return parent;
10. }
11. TreeNode *sortedListToBST(ListNode *head) {
12. if(head==NULL) return NULL;
13. ListNode *p=head;
14. int cnt=0;
15. while(p!=NULL) {
16. cnt++;
17. p=p->next;
18. }
19. return addNode(head,0,cnt-1);
20. }