Leetcode 之Convert Sorted List to Binary Search Tree（55）

和上题思路基本一致，不同的地方在于，链表不能随机访问中间元素。

int listLength(ListNode* node)
      {
          int n = ;
          while (node)
          {
              n++;
              node = node->next;
          }
          return n;
      }
      ListNode* nth_node(ListNode* node, int n)
      {
          while (--n)node = node->next;
          return node;
      }
      TreeNode* sortedListToBST(ListNode* head)
      {
          sortedListToBST(head, listLength(head));
      }
      TreeNode* sortedListToBST(ListNode* head, int len)
      {
          if (len == )return nullptr;
          if (len == )return new TreeNode(head->val);

          TreeNode* root = new TreeNode(nth_node(head, len /  + )->val);
          root->left = sortedListToBST(head, len / );
          root->right = sortedListToBST(head, len /  + );

          return root;
      }