LeetCode 783. Minimum Distance Between BST Nodes - Weekly Contest 71

https://leetcode.com/contest/weekly-contest-71/problems/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.

	/**
	* Definition for a binary tree node.
	* public class TreeNode {
	* int val;
	* TreeNode left;
	* TreeNode right;
	* TreeNode(int x) { val = x; }
	* }
	*/
	class Solution {
	public int minDiffInBST(TreeNode root) {
	// filter abnormal cases
	if (root == null) {
	return 0;
	}
	nodes = new LinkedList<>();
	helper(root);
	// return the final result
	int answer = Integer.MAX_VALUE;
	int slow = nodes.getFirst();
	for (int i = 1; i < nodes.size(); i++) {
	int fast = nodes.get(i);
	answer = Math.min(answer, Math.abs(fast - slow));
	slow = fast;
	}
	return answer;
	}
	LinkedList<Integer> nodes;
	public void helper(TreeNode root) {
	if (root == null) {
	return;
	}
	helper(root.left);
	nodes.add(root.val);
	helper(root.right);
	}
	}

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