Longest Arithmetic Subsequence LeetCode solution in Java

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class Solution {
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  public int longestArithSeqLength(int[] nums) {
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    int n = nums.length;
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    int longest = 0;
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    Map<Integer, Integer>[] dp = new HashMap[n];
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    for (int i = 0; i < n; i++) {
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      dp[i] = new HashMap<>();
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      for (int j = 0; j < i; j++) {
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        int diff = nums[i] - nums[j];
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        dp[i].put(diff, dp[j].getOrDefault(diff, 1) + 1);
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        longest = Math.max(longest, dp[i].get(diff));
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      }
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    }
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    return longest;
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  }
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}