二路归并排序

public class BinaMergeSort {
	// 将数组a[start..mid],a[mid+1..end]合并到数组union
	static void Merge(int[] a, int start, int mid, int end, int[] union) {
		// 左边数组起始下标
		int left = start;
		// 右边数组起始下标
		int right = mid + 1;
		// 合并到union的起始下标
		int uPos = start;
		for (; left <= mid && right <= end;) {
			if (a[left] > a[right]) {
				union[uPos++] = a[right++];
			} else {
				union[uPos++] = a[left++];
			}
		}
		while (left <= mid) {
			union[uPos++] = a[left++];
		}
		while (right <= end) {
			union[uPos++] = a[right++];
		}
		//合并好后再将其赋给a
		for(int i=start;i<=end;i++){
			a[i] = union[i];
		}
	}

	static void MergeSort(int a[], int low, int high, int[] union) {
		if (low < high) {
			int mid = (low + high) / 2;
			MergeSort(a, low, mid, union);
			MergeSort(a, mid + 1, high, union);
			Merge(a, low, mid, high, union);
		}
	}

	public static void main(String[] args){
		int[] a = { 5, 3, 2, 0, 8, 4, 6, 7, 1, 4 };
		//union为辅助数组
		int[] union = new int[a.length];
		MergeSort(a, 0, a.length - 1, union);
		for (int i : a) {
			System.out.print(i + " ");
		}
	}
}