Merge Sort

MergeSort.java

void mergesort(int[] array) {		
		int[] helper = new int[array.length];
		mergesort(array, helper, 0, array.length - 1);
	}
	
	void mergesort(int[] array, int[] helper, int low, int high) {
		if (low < high) {
			int middle = (low + high) / 2;
			mergesort(array, helper, low, middle); // left
			mergesort(array, helper, middle + 1, high); // right 
			merge(array, helper, low, middle, high);
		}
	}
	
	void merge(int[] array, int[] helper, int low, int middle, int high) {
		//We can only store first half of the array if we want.
		
		for (int i = low; i <= high; i++) {
			helper[i] = array[i]; 
		}
		int helperLeft = low;
		int helperRight = middle + 1;
		int current = low;
		
		while (helperLeft <= middle && helperRight <= high) {
			array[current++] = helper[helperLeft] < helper[helperRight] 
					? helper[helperLeft++] 
					: helper[helperRight++];
		}
		
		while (helperLeft <= middle) {
			array[current++] = helper[helperLeft++];
		}
		
	}