Minimum Distance Between BST Nodes LeetCode solution in Java

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/**
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 * Definition for a binary tree node. public class TreeNode { int val; TreeNode left; TreeNode
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 * right; TreeNode() {} TreeNode(int val) { this.val = val; } TreeNode(int val, TreeNode left,
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 * TreeNode right) { this.val = val; this.left = left; this.right = right; } }
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 */
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class Solution {
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  int mini = Integer.MAX_VALUE;
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  public void find(TreeNode root, ArrayList<Integer> arr) {
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    if (root == null) {
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      return;
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    }
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    arr.add(root.val);
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    find(root.left, arr);
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    for (int i = arr.size() - 2; i >= 0; i--) {
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      mini = Math.min(mini, Math.abs(root.val - arr.get(i)));
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    }
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    find(root.right, arr);
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    arr.remove(arr.size() - 1);
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  }
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  public int minDiffInBST(TreeNode root) {
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    ArrayList<Integer> arr = new ArrayList<>();
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    find(root, arr);
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    return mini;
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  }
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}