递归的把一个有序单向链表转化成二叉排序树
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
/**
* Definition for a binary tree node.
* struct TreeNode {
* int val;
* TreeNode *left;
* TreeNode *right;
* TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
TreeNode* sortedListToBSTHelper(ListNode* head, ListNode* tail) {
ListNode* slow = head;
ListNode* fast = head;
if(head==tail) return nullptr;
while(fast!=tail && fast->next!=tail){
fast = fast->next->next;
slow = slow->next;
}
TreeNode* thead = new TreeNode(slow->val);
thead->left = sortedListToBSTHelper(head,slow);
thead->right = sortedListToBSTHelper(slow->next,tail);
return thead;
}
TreeNode* sortedListToBST(ListNode* head) {
if (head == nullptr) {
return nullptr;
} else {
return sortedListToBSTHelper(head, nullptr);
}
}
};