0442. Find All Duplicates in an Array

https://leetcode.com/problems/find-all-duplicates-in-an-array

Description

Given an integer array nums of length n where all the integers of nums are in the range [1, n] and each integer appears once or twice, return an array of all the integers that appears twice.

You must write an algorithm that runs in O(n)time and uses only constant extra space.

Example 1:

**Input:** nums = [4,3,2,7,8,2,3,1]
**Output:** [2,3]

Example 2:

**Input:** nums = [1,1,2]
**Output:** [1]

Example 3:

**Input:** nums = [1]
**Output:** []

Constraints:

• n == nums.length

• 1 <= n <= 105

• 1 <= nums[i] <= n

• Each element in nums appears once or twice.

ac

class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        List<Integer> res = new ArrayList<Integer>();
        // corner cases
        if (nums.length == 0) return res;

        // iterate 
        int n = nums.length;
        for (int i = 0; i < n; i++) {
            while (nums[i] < n && i != nums[i] && nums[nums[i]] != nums[i]) {
                swap(nums, i, nums[i]);
            }
            // special case, n
            if (nums[i] == n) swap(nums, 0, i);
        }

        // final check get result
        if (nums[0] != n) res.add(nums[0]);
        for (int i = 1; i < n; i++) {
            if (nums[i] != i) {
                res.add(nums[i]);
            }
        }

        return res;
    }

    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}
/* same code as 448*/