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 = -1 ;
    int minDiffInBST(TreeNode* root) {
        if (root->left != NULL)
            minDiffInBST(root->left) ; 
        if (pre >= 0)
            res = min(res , root->val - pre) ;
        pre = root->val ;
        if (root->right != NULL)
            minDiffInBST(root->right) ; 
        return res ;
    }
};