LeetCode-in-Java

442. Find All Duplicates in an Array

Medium

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.

Solution

import java.util.ArrayList;
import java.util.List;

public class Solution {
    public List<Integer> findDuplicates(int[] nums) {
        int num = nums.length;
        int[] count = new int[num];
        List<Integer> list = new ArrayList<>();
        for (int j : nums) {
            // count number of times each number appears
            count[j - 1] += 1;
        }
        for (int i = 0; i < num; i++) {
            // if count of num is 2 add pos to list
            if (count[i] == 2) {
                list.add(i + 1);
            }
        }
        return list;
    }
}

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