L783.java

class Solution783 {
    class Solution {

        private int minDiff = Integer.MAX_VALUE;
        private TreeNode previous;

        /**
         * 783. Minimum Distance Between BST Nodes https://leetcode.com/problems/minimum-distance-between-bst-nodes/description/
         * Inorder traversal of a BST processes the nodes in sorted order. We maintain the previously seen element while traversing
         * and compare with min seen so far to keep track of the min difference.
         *
         * @param root TreeNode
         *             Root of the tree
         * @return Min difference between any two nodes
         * @timeComplexity O(n) where n is the number of nodes in tree
         * @spaceComplexity O(1)
         */
        public int minDiffInBST(TreeNode root) {
            if (root == null) {
                return minDiff;
            }
            // Traverse left
            minDiffInBST(root.left);

            // Traverse root
            if (previous != null) {
                minDiff = Math.min(minDiff, root.val - previous.val);
            }
            previous = root;

            // Traverse right
            minDiffInBST(root.right);

            return minDiff;
        }
    }
}