783. Minimum Distance Between BST Nodes

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: 1
Explanation:
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）：

 /**
  * 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:
     int res = INT_MAX ;
     int pre = - ;
     int minDiffInBST(TreeNode* root) {
         if (root->left != NULL)
             minDiffInBST(root->left) ;
         if (pre >= )
             res = min(res , root->val - pre) ;
         pre = root->val ;
         if (root->right != NULL)
             minDiffInBST(root->right) ;
         return res ;
     }
 };