Number of Good Leaf Nodes Pairs LeetCode solution in Java

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class Solution {
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  static int res;
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  public int countPairs(TreeNode root, int distance) {
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    res = 0;
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    rec(root, distance);
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    return res;
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  }
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  static List<Integer> rec(TreeNode root, int dist) {
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    if (root == null) {
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      return new LinkedList<Integer>();
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    }
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    List<Integer> left = rec(root.left, dist);
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    List<Integer> right = rec(root.right, dist);
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    if (left.size() == 0 && right.size() == 0) {
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      List<Integer> temp = new LinkedList<>();
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      temp.add(1);
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      return temp;
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    }
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    for (int i : left) {
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      for (int j : right) {
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        if (i + j <= dist) {
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          res++;
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        }
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      }
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    }
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    List<Integer> temp = new LinkedList<>();
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    for (int i : left) {
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      temp.add(i + 1);
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    }
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    for (int i : right) {
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      temp.add(i + 1);
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    }
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    return temp;
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  }
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}