排序算法（五）归并排序

基本思想：归并（Merge）排序法是将两个（或两个以上）有序表合并成一个新的有序表，即把待排序序列分为若干个子序列，每个子序列是有序的。然后再把有序子序列合并为整体有序序列

	// 归并排序
	public static void mergesort(int[] arr, int left, int right, boolean reverse) {
		if (left >= right || arr == null || arr.length <= 1) {
			return;
		}
		int mid = (left + right) / 2;
		mergesort(arr, left, mid, reverse);
		mergesort(arr, mid + 1, right, reverse);
		merge(arr, left, mid, right, reverse);
	}
	public static void merge(int[] arr, int left, int mid, int right, boolean reverse) {
		int[] temp = new int[right - left + 1];
		int l = left;
		int r = mid + 1;
		int n = 0;
		while (l <= mid && r <= right) {
			if (reverse) {
				if (arr[l] < arr[r]) {
					temp[n++] = arr[l++];
				} else {
					temp[n++] = arr[r++];
				}
			} else {
				if (arr[l] > arr[r]) {
					temp[n++] = arr[l++];
				} else {
					temp[n++] = arr[r++];
				}
			}
		}
		while (l <= mid) {
			temp[n++] = arr[l++];
		}
		while (r <= right) {
			temp[n++] = arr[r++];
		}
		for (int i = 0; i < temp.length; i++) {
			arr[i + left] = temp[i];
		}
	}