783. minimum distance between bst nodes

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.

public int minDiffInBST(TreeNode root) {
    Stack<TreeNode> stack = new Stack<>();
    TreeNode cur = root;
    int prev = 0, min = Integer.MAX_VALUE, count = 0;
    while (cur != null || !stack.isEmpty()) {
        while (cur != null) {
            stack.push(cur);
            cur = cur.left;
        }
        cur = stack.pop();
        if (count != 0) {
            int diff =  cur.val - prev;
            if (diff < min) min = diff;
        }
        count++;
        prev = cur.val;
        cur = cur.right;
    }
    return min;
}