Rearrange array in alternating positive & negative items with O(1) extra space in C++

We are given an integer type array containing both positive and negative numbers, let's say, arr[] of any given size. The task is to rearrange an array in such a manner that there will be a positive number that will be surrounded by negative numbers. If there are more positive and negative numbers, then they will be arranged at the end of an array.

Let us see various input output scenarios for this −

Input − int arr[] = {-1, -2, -3, 1, 2, 3}

Output − Array before Arrangement: -1 -2 -3 1 2 3 Rearrangement of an array in alternating positive & negative items with O(1) extra space is: -1 1 -2 2 -3 3

Explanation − we are given an integer array of size 6 containing both positive and negative elements. Now, we will rearrange the array in such a manner that all the positive elements will be surrounded by the negative elements and all the extra elements will be added in the end of an array i.e -1 1 -2 2 -3 3 will be the final result.

Input − int arr[] = {-1, -2, -3, 1, 2, 3, 5, 5, -5, 3, 1, 1};

Output − Array before Arrangement: -1 -2 -3 1 2 3 5 5 -5 3 1 1 Rearrangement of an array in alternating positive & negative items with O(1) extra space is: -1 1 -2 2 -3 3 -5 5 5 3 1 1

Explanation − we are given an integer array of size 12 containing both positive and negative elements. Now, we will rearrange the array in such a manner that all the positive elements will be surrounded by the negative elements and all the extra elements will be added in the end of an array i.e -1 1 -2 2 -3 3 -5 5 5 3 1 1 will be the final result.

Approach used in the below program is as follows

• Input an array of integer type elements and calculate the size of an array.

• Print an array before performing the rearrangement action using the FOR loop.

• Call to the function Rearrangement(arr, size) by passing array and size of an array as a parameter.

• Inside the function Rearrangement(arr, size)

• Declare an integer variable ‘ptr’ and initialize it with -1.

• Start loop FOR from i to 0 till i less than size. Inside the loop, check IF ptr greater than 0 then check IF arr[i] greater than 0 AND arr[ptr] less than 0 OR arr[i] less than 0 AND arr[ptr] greater than 0 then call to the function move_array(arr, size, ptr, i) and check IF i - ptr greater than 2 then set ptr to ptr + 2. ELSE, set ptr to -1.

• Check IF ptr to -1 then check arr[i] greater than 0 AND !(i & 0x01) OR (arr[i] less than 0) AND (i & 0x01) then set ptr to i.

• Inside the function move_array(int arr[], int size, int ptr, int temp)

• Declare a variable as ‘ch’ of type character and set it with arr[temp].

• Start loop FOR from i to temp till i greater than ptr. Inside the loop, set arr[i] with arr[i - 1].

• Set arr[ptr] to ch.

Example

#include <iostream>
#include <assert.h>
using namespace std;
void move_array(int arr[], int size, int ptr, int temp){
char ch = arr[temp];
for(int i = temp; i > ptr; i--){
arr[i] = arr[i - 1];
}
arr[ptr] = ch;
}
void Rearrangement(int arr[], int size){
int ptr = -1;
for(int i = 0; i < size; i++){
if (ptr >= 0){
if(((arr[i] >= 0) && (arr[ptr] < 0)) || ((arr[i] < 0) && (arr[ptr] >= 0))){
move_array(arr, size, ptr, i);
if(i - ptr >= 2){
ptr = ptr + 2;
}
else{
ptr = -1;
}
}
}
if(ptr == -1){
if (((arr[i] >= 0) && (!(i & 0x01))) || ((arr[i] < 0) && (i & 0x01))){
ptr = i;
}
}
}
}
int main(){
//input an array
int arr[] = {-1, -2, -3, 1, 2, 3};
int size = sizeof(arr) / sizeof(arr[0]);
//print the original Array
cout<<"Array before Arrangement: ";
for (int i = 0; i < size; i++){
cout << arr[i] << " ";
}
//calling the function to rearrange the array
Rearrangement(arr, size);
//print the array after rearranging the values
cout<<"\nRearrangement of an array in alternating positive & negative items with O(1) extra space is: ";
for(int i = 0; i < size; i++){
cout<< arr[i] << " ";
}
return 0;
}

Output

If we run the above code it will generate the following Output

Array before Arrangement: -1 -2 -3 1 2 3
Rearrangement of an array in alternating positive & negative items with O(1) extra space is: -1 1 -2 2 -3 3

Updated on: 02-Nov-2021

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