Given a Binary Search Tree (BST) with the root node root, return the minimum difference between the values of any two different nodes in the tree.

Example :

Input: root = [4,2,6,1,3,null,null]Output: 1Explanation:Note that root is a TreeNode object, not an array.The given tree [4,2,6,1,3,null,null] is represented by the following diagram:          4        /   \      2      6     / \        1   3  while the minimum difference in this tree is 1, it occurs between node 1 and node 2, also between node 3 and node 2. 求二叉搜索树任意两节点之间的差值，要求最小 C++（4ms）：

1 /** 2  * Definition for a binary tree node. 3  * struct TreeNode { 4  *     int val; 5  *     TreeNode *left; 6  *     TreeNode *right; 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8  * }; 9  */10 class Solution {11 public:12     int res = INT_MAX ;13     int pre = -1 ;14     int minDiffInBST(TreeNode* root) {15         if (root->left != NULL)16             minDiffInBST(root->left) ; 17         if (pre >= 0)18             res = min(res , root->val - pre) ;19         pre = root->val ;20         if (root->right != NULL)21             minDiffInBST(root->right) ; 22         return res ;23     }24 };