LeetCode-109-Convert Sorted List to Binary Search Tree

算法描述：

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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example:

Given the sorted linked list: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

解题思路：理解原理

    TreeNode* sortedListToBST(ListNode* head) {
        if(head == nullptr) return nullptr;
        return helper(head,nullptr);       
    }
    
    TreeNode* helper(ListNode* start, ListNode* end){
         if(start==end) return nullptr;
        ListNode* fast = start;
        ListNode* slow = start;
        while(fast->next!=end && fast->next->next!=end){
            fast = fast->next->next;
            slow = slow->next;
        }
        
        TreeNode* node = new TreeNode(slow->val);
        node->left = helper(start,slow);
        node->right = helper(slow->next,end);
        return node;
    }