MINIMUM DISTANCE BETWEEN BST NODES.cpp

class Solution
{
public:
     void helper(TreeNode *root, int &res, int &pre)
     {
          if (!root)
               return;
          helper(root->left, res, pre);
          if (pre != -1)
               res = min(res, root->val - pre);
          pre = root->val;
          helper(root->right, res, pre);
     }
     int minDiffInBST(TreeNode *root)
     {
          int res = INT_MAX;
          int pre = -1;
          helper(root, res, pre);
          return res;
     }
};

// LEETCODE - 783