442. Find All Duplicates in an Array

Flip

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

        for (int i = 0; i < nums.length; i++) {
            int abs = Math.abs(nums[i]);
            if (nums[abs - 1] < 0) {
                // 已经被翻转过，收集该数字
                list.add(abs);
            } else {
                nums[abs - 1] = -nums[abs - 1];
            }
        }

        return list;
    }
}

Hash

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

        for (int i = 0; i < nums.length; i++) {
            int targetIndex = nums[i] - 1;
            while (nums[i] != nums[targetIndex]) {
                swap(nums, i, targetIndex);
                targetIndex = nums[i] - 1;
            }
        }

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

        return numList;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}

References

442. Find All Duplicates in an Array