Geeks For Geeks Reverse Array

 Clear------ 0.40 sec

Approach 1 : O(n) O(n) taking additional array

public static void main(String arg[]){

Scanner sc=new Scanner(System.in);

                   int t=sc.nextInt();

               while(t -->0){

System.out.println("Enter size of Array : ");

int n=sc.nextInt();

int arr[]=new int[n];

System.out.println("Enter Elements : ");

for(int i=0;i<n;i++){

arr[i]=sc.nextInt();

}

System.out.println("Actual Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr[i]+" ");

}

System.out.println();

int index=0;

int arr1[]=new int[n];

for(int i=n-1;i>-1;i--){

arr1[index]=arr[i];

index++;

}

System.out.println("Reversed Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr1[i]+" ");

}

            System.out.println();

}

}

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Cleared 0.37 sec

Approach 2 O(n) O(1) swapping by double pointers (one from start and one from end)

public static void main(String arg[]){

Scanner sc=new Scanner(System.in);

                   int t=sc.nextInt();

               while(t -->0){

                System.out.println("Enter size of Array : ");

int n=sc.nextInt();

int arr[]=new int[n];

System.out.println("Enter Elements : ");

for(int i=0;i<n;i++){

arr[i]=sc.nextInt();

}

System.out.println("Actual Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr[i]+" ");

}

System.out.println();

int start=0;

int end=n-1;

while(start<end){

int temp=0;

temp=arr[end];

arr[end]=arr[start];

arr[start]=temp;

start++;

end--;

}

System.out.println("Reversed Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr[i]+" ");

}

           System.out.println();

        }

    }

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Cleared 0.47 sec 

Approach 3 O(n) O(n) ArrayList and Collections.reverse

public static void main(String arg[]){

Scanner sc=new Scanner(System.in);

            int t =sc.nextInt();

            while(t -->0){

System.out.println("Enter size of Array : ");

int n=sc.nextInt();

int arr[]=new int[n];

System.out.println("Enter Elements : ");

for(int i=0;i<n;i++){

arr[i]=sc.nextInt();

}

System.out.println("Actual Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr[i]+" ");

}

System.out.println();

ArrayList<Integer> aa=new ArrayList<Integer>();

for(int i=0;i<arr.length;i++){

aa.add(arr[i]);

}

Collections.reverse(aa);

for(int i=0;i<aa.size();i++){

System.out.print(aa.get(i)+" ");

}

System.out.println();

}

}

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Cleared 0.40 sec 

Approach 4 O(n) O(1) swapping till half

public static void main(String arg[]){

Scanner sc=new Scanner(System.in);

                int t=sc.nextInt();

                while(t -->0){

System.out.println("Enter size of Array : ");

int n=sc.nextInt();

int arr[]=new int[n];

System.out.println("Enter Elements : ");

for(int i=0;i<n;i++){

arr[i]=sc.nextInt();

}

System.out.println("Actual Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr[i]+" ");

}

System.out.println();

for(int i=0;i<n/2;i++){

int temp=0;

temp=arr[n-1-i];

arr[n-1-i]=arr[i];

arr[i]=temp;

}

System.out.println("Reverse Array is : ");

for(int i=0;i<n;i++){

System.out.print(arr[i]+" ");

}

System.out.println();

}

    }