19. Find All Duplicates in an Array

Topic :

arrays

Difficulty :

medium

Problem Link :

problem statement

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.io.*;
import java.util.*;
 class Find_All_Duplicates
{
   public static void main (String args[])
   {
       int nums[]={4,3,2,7,8,2,3,1};
       System.out.println(findDuplicates(nums));
    }
     static List<Integer> findDuplicates(int[] nums) {
        
        List <Integer> ans= new ArrayList<>();
           int i=0;    
        while(i<nums.length)
        {
            int correctIndex=nums[i]-1;
            if(nums[correctIndex]!=nums[i])
            {
                int temp=nums[correctIndex];
                nums[correctIndex]=nums[i];
                nums[i]=temp;
            }
            else
            {
                i++;
            }
        }
        
        for(int ind=0;ind<nums.length;ind++)
        {
            if(nums[ind]!=ind+1)
                ans.add(nums[ind]);
        }
        
        return ans;
    }
}