Count Number of Bad Pairs LeetCode solution in Java

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class Solution {
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  public long countBadPairs(int[] nums) {
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    HashMap<Integer, Integer> seen = new HashMap<>();
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    long count = 0;
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    for (int i = 0; i < nums.length; i++) {
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      int diff = i - nums[i];
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      if (seen.containsKey(diff)) {
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        count += (i - seen.get(diff));
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      } else {
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        count += i;
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      }
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      seen.put(diff, seen.getOrDefault(diff, 0) + 1);
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    }
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    return count;
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  }
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}