Rearrange positive and negative numbers

An array contains both positive and negative numbers in random order. Rearrange the array elements so that positive and negative numbers are placed alternatively. Number of positive and negative numbers need not be equal. If there are more positive numbers they appear at the end of the array. If there are more negative numbers, they too appear in the end of the array.

For example, if the input array is [-1, 2, -3, 4, 5, 6, -7, 8, 9], then the output should be [9, -7, 8, -3, 5, -1, 2, 4, 6]

Strategy: Partition in Quick Sort
http://www.geeksforgeeks.org/rearrange-positive-and-negative-numbers-publish/

Variation 2:
Given an array of positive and negative numbers, arrange them such that all negative integers appear before all the positive integers in the array without using any additional data structure like hash table, arrays, etc. The order of appearance should be maintained.

Examples:

Input:  [12 11 -13 -5 6 -7 5 -3 -6]
Output: [-13 -5 -7 -3 -6 12 11 6 5]
http://www.geeksforgeeks.org/rearrange-positive-and-negative-numbers/