Day-6 Find All Duplicates in an Array

class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> ans = new ArrayList<>();

        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[j] == nums[i]) {
                    ans.add(nums[i]);
                    break;
                }
            }
        }

        return ans;
    }
}

class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> ans = new ArrayList<>();

        Arrays.sort(nums);

        for (int i = 1; i < nums.length; i++) {
            if (nums[i] == nums[i - 1]) {
                ans.add(nums[i]);
                i++;        // skip over next element
            }
        }

        return ans;
    }
}


class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> ans = new ArrayList<>();
        Set<Integer> seen = new HashSet<>();

        for (int num : nums) {
            if (seen.contains(num)) {
                ans.add(num);
            } else {
                seen.add(num);
            }
        }

        return ans;
    }
}

class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> ans = new ArrayList<>();

        for (int num : nums) {
            nums[Math.abs(num) - 1] *= -1;
        }

        for (int num : nums) {
            if (nums[Math.abs(num) - 1] > 0) {
                ans.add(Math.abs(num));
                nums[Math.abs(num) - 1] *= -1;
            }
        }

        return ans;
    }
}

class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> ans = new ArrayList<>();

        for (int num : nums) {
            if (nums[Math.abs(num) - 1] < 0) { // seen before
                ans.add(Math.abs(num));
            }
            nums[Math.abs(num) - 1] *= -1;
        }

        return ans;
    }
}