Find All Duplicates in an Array LeetCode solution in Java

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class Solution {
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  public List<Integer> findDuplicates(int[] nums) {
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    List<Integer> ans = new ArrayList<>();
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    for (int i = 0; i < nums.length; i++) {
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      int ind = Math.abs(nums[i]) - 1;
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      if (nums[ind] < 0) {
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        ans.add(Math.abs(nums[i]));
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      } else {
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        nums[ind] = -1 * nums[ind];
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      }
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    }
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    return ans;
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  }
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}