public class Solution {
    public int minDiffInBST(TreeNode root) {
        int minDiff = Integer.MAX_VALUE;
        TreeNode prev = null;
        
        Stack<TreeNode> stack = new Stack<>();
        TreeNode curr = root;

        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }

            curr = stack.pop();

            if (prev != null) {
                minDiff = Math.min(curr.val - prev.val, minDiff);
            }
            
            prev = curr;
            
            curr = curr.right;
        }

        return minDiff;
    }
}