Find All Numbers Disappeared in an Array LeetCode solution in Java

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class Solution {
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  public List<Integer> findDisappearedNumbers(int[] nums) {
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    List<Integer> res = new ArrayList<>();
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    // 0 1 2 3 4 5 6 7 <- indx
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    // 4 3 2 7 8 2 3 1 <- nums[i]
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    for (int i = 0; i < nums.length; i++) {
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      int indx = Math.abs(nums[i]) - 1;
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      if (nums[indx] > 0) {
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        nums[indx] = nums[indx] * -1;
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      }
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    }
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    // 0 1 2 3 4 5 6 7 <- indx
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    // -4 -3 -2 -7 8 2 -3 -1 <- nums[i]
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    for (int i = 0; i < nums.length; i++) {
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      if (nums[i] > 0) {
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        res.add(i + 1);
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      } else {
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        nums[i] *= -1;
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      }
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    }
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    // [ 5, 6]
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    return res;
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  }
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}