leetcode-109. 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.

思路：才用快慢指针，找到中间节点，然后递归处理。

accepted code：

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