Write a function that performs ONE PARTITION for an array of integers, as defined by the quicksort algorithm. The function has three parameters: an array of integers, the index of the leftmost element to be partitioned, and the index of the rightmost element to be partitioned. (See the quicksort notes for more explanation.) The return value is the index where the pivot element is placed after the partition. When the function returns, all elements to the left of that location (within the partitioned portion of the array) must be less than or equal to the pivot, and all elements to the right of that location (within the partitioned portion of the array) must be greater than or equal to the pivot. int partition(int a[], int left, int right);