Minimum Distance Between BST Nodes

找BST上两个node之间的最小值, 因为是bst所以已经是sorted了, 直接比较就可以

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