Leetcode 之Convert Sorted Array to Binary Search Tree（54）

思路很简单，用二分法，每次选中间的点作为根结点，用左、右结点递归。

TreeNode* sortedArrayToBST(vector<int> &num)
      {
          return sortedArrayToBST(num.begin(), num.end());
      }
      template<typename RandomAccessIterator>
      TreeNode* sortedArrayToBST(RandomAccessIterator first, RandomAccessIterator last)
      {
          const auto length = distance(first, last);
          if (length <= 0)return nullptr;

          auto mid = first + length / 2;
          TreeNode *root = new TreeNode(*mid);
          root->left = sortedArrayToBST(first, mid);
          root->right = sortedArrayToBST(mid, last);

          return root;
      }