109.cpp

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