Arrays-DS HackerRank Solution | By KodeSrc

In this post we will solve the Arrays-DS HackerRank Question

Problem Statement:  We are given an array of integers like A[4,5,6] we need to reverse this array like A[6,5,4].

This question is easy and could have more than 1 solution . So probably we will not need much explanation but we will discuss different approaches in this post to get better understanding.

    Data Set:

    A=[10,4,1,6,9,3,2]

    Expected Output:
    
    [2,3,9,6,1,4,10]

  

Approach 1: Create new List and add the elements of the existing list in reverse order and return the new list.

  

public static List<Integer> reverseArray(List<Integer> a) {

       List<Integer> reversedList=new ArrayList<>();

       for(int i=a.size()-1;i>=0;i--){
           reversedList.add(a.get(i));
       }

       return reversedList;

    }


Input:

[1 ,22 ,23 ,57 ,67]
 
Output:

[67 ,57 ,23 ,22 ,1] 

  

Note: Disadvantage of this Approach is that it will create a new list and occupy memory , so this solution will have O(n) space complexity.

Approach 2: Use the index of the elements and swap the elements of the list.

Basically Create 2 variables that will point to the start and end index of the list , now swap / interchange the elements at that positions and increment the start index and decrement the end index and continue the same process until all the elements are swapped and at right position.

  

       public static List<Integer> reverseArray(List<Integer> a) {

        int startIndex = 0;
        int endIndex = a.size() - 1;

        while (startIndex < endIndex) {
            int elementAtStartIndex = a.get(startIndex);
            int elementAtEndIndex = a.get(endIndex);

            a.set(startIndex, elementAtEndIndex);
            a.set(endIndex, elementAtStartIndex);
            startIndex++;
            endIndex--;
        }
        return a;

    }


Input:

[1 ,22 ,23 ,57 ,67]
 
Output:

[67 ,57 ,23 ,22 ,1] 


  

Additionally we can also use Collections swap() method for swapping the values at a particular index instead of storing the values in local variables and assigning it at particular indexes.

  
   public static List<Integer> reverseArray(List<Integer> a) {

        int startIndex = 0;
        int endIndex = a.size() - 1;

        while (startIndex < endIndex) {
            Collections.swap(a,startIndex,endIndex);
            startIndex++;
            endIndex--;
        }
        return a;

    }


Input:

[1 ,22 ,23 ,57 ,67]
 
Output:

[67 ,57 ,23 ,22 ,1] 



  

Or we  can write our own  basic swap function as well.

  
      public static List<Integer> swapElements(List<Integer> list, int i, int j) {
        int temp = list.get(i);
        list.set(i, list.get(j));
        list.set(j, temp);
        return list;
    }


  

Finally its up to you what you have to use.

Output:

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