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.

class Solution {
    int min = Integer.MAX_VALUE;
    TreeNode pre = null;
    public int minDiffInBST(TreeNode root) {
        if(root == null) return min;
        
        minDiffInBST(root.left);
        if(pre != null) min = Math.min(min, root.val - pre.val);
        pre = root;
        minDiffInBST(root.right);
        return min;
    }
}