Rearrange positive and negative numbers in O(n) time and O(1) complexity. If there are more positive or negative number, they will appear at the end of the array.
Ex:
Input Array : {-1,2,-3,4,5,6,-7,8,9}
Output Array: {9,-7,8,-3,5,-1,2,4,6}
Ex:
Input Array : {-1,2,-3,4,5,6,-7,8,9}
Output Array: {9,-7,8,-3,5,-1,2,4,6}
The solution is to first separate positive and negative numbers using partition process of QuickSort. In the partition process, consider 0 as value of pivot element so that all negative numbers are placed before positive numbers. Once negative and positive numbers are separated, we start from the first negative number and first positive number, and swap every alternate negative number with next positive number.
ReplyDelete// A C++ program to put positive numbers at even indexes (0, 2, 4,..)
// and negative numbers at odd indexes (1, 3, 5, ..)
#include
// prototype for swap
void swap(int *a, int *b);
// The main function that rearranges elements of given array. It puts
// positive elements at even indexes (0, 2, ..) and negative numbers at
// odd indexes (1, 3, ..).
void rearrange(int arr[], int n)
{
// The following few lines are similar to partition process
// of QuickSort. The idea is to consider 0 as pivot and
// divide the array around it.
int i = -1,j;
for (j = 0; j < n; j++)
{
if (arr[j] < 0)
{
i++;
swap(&arr[i], &arr[j]);
}
}
// Now all positive numbers are at end and negative numbers at
// the beginning of array. Initialize indexes for starting point
// of positive and negative numbers to be swapped
int pos = i+1, neg = 0;
// Increment the negative index by 2 and positive index by 1, i.e.,
// swap every alternate negative number with next positive number
while (pos < n && neg < pos && arr[neg] < 0)
{
swap(&arr[neg], &arr[pos]);
pos++;
neg += 2;
}
}
// A utility function to swap two elements
void swap(int *a, int *b)
{
int temp = *a;
*a = *b;
*b = temp;
}
// A utility function to print an array
void printArray(int arr[], int n)
{
int i;
for (i = 0; i < n; i++)
printf("%4d ", arr[i]);
}
// Driver program to test above functions
int main()
{
int arr[] = {-1, 2, -3, 4, 5, 6, -7, 8, 9};
int n = sizeof(arr)/sizeof(arr[0]);
rearrange(arr, n);
printArray(arr, n);
return 0;
}