用java实现的合并排序

MergeSort

package first;

/**
 * 合并排序，时间代价为O(n)，总的计算次数是一定的，每次比较花得时间是个常量<br/>
 * 但是前提是左右两个数组是排好序的
 * 
 * @author weizhankui
 * @date 2014-7-27
 *
 */

public class Sort {
	/*
	 * 合并排序（降序）
	 * 
	 * a是数组 p,q,r是下标，企业满足p<=q<r 假设a[p..q]和a[q+1..r]都是排好序的
	 */
	public static void merge_desc(int[] a, int p, int q, int r) {
		int n1 = q - p + 1;
		int n2 = r - q;
		int[] left = new int[n1];
		int[] right = new int[n2];
		for (int i = 0; i < n1; i++) {
			left[i] = a[p + i];
		}
		for (int j = 0; j < n2; j++) {
			right[j] = a[q + j + 1];
		}

		int i = 0;
		int j = 0;
		for (int k = p; k <= r; k++) {
			if (i == n1) {
				a[k] = right[j];
				j++;
			} else if (j == n2) {
				a[k] = left[i];
				i++;
			} else {
				if (left[i] >= right[j]) {
					a[k] = left[i];
					i++;
				} else {
					a[k] = right[j];
					j++;
				}
			}
		}

		printArray(a);
	}

	/*
	 * 合并排序（升序）
	 * 
	 * a是数组 p,q,r是下标，企业满足p<=q<r 假设a[p..q]和a[q+1..r]都是排好序的
	 */
	public static void merge_asc(int[] a, int p, int q, int r) {
		int n1 = q - p + 1;
		int n2 = r - q;
		int[] left = new int[n1];
		int[] right = new int[n2];
		for (int i = 0; i < n1; i++) {
			left[i] = a[p + i];
		}
		for (int j = 0; j < n2; j++) {
			right[j] = a[q + j + 1];
		}

		int i = 0;
		int j = 0;
		for (int k = p; k <= r; k++) {
			if (i == n1) {
				a[k] = right[j];
				j++;
			} else if (j == n2) {
				a[k] = left[i];
				i++;
			} else {
				if (left[i] <= right[j]) {
					a[k] = left[i];
					i++;
				} else {
					a[k] = right[j];
					j++;
				}
			}
		}

		printArray(a);
	}

	public static void main(String[] args) {

		int[] a = { 9, 5, 1, 8, 7, 6, 4, 3, 2 };
		System.out.println("降序排列前：");
		printArray(a);
		System.out.println("降序排列后：");
		merge_desc(a, 0, 2, 8);

		a = new int[] { 1, 5, 6, 9, 2, 3, 4, 6, 7, 8 };
		System.out.println("升序排列前：");
		printArray(a);
		System.out.println("升序排列后：");
		merge_asc(a, 0, 3, 8);

	}

	public static void printArray(int[] a) {
		for (int num : a) {
			System.out.print(num + ",");
		}
		System.out.println("");
	}

}

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