归并排序

归并排序：算法将数组分为两半，对每部分递归地应用归并排序。在两部分都排好序后，对它们进行归并。即使用分治法对数组排序。

/*归并排序程序清单*/
public class MergeSort {

	public static void mergeSort(int[] list){
		if(list.length>1){
			int[] firstHalf=new int[list.length/2];
			System.arraycopy(list, 0, firstHalf, 0, list.length/2);
			mergeSort(firstHalf);
			int secondHalfLength=list.length-list.length/2;
			int[] secondHalf=new int[secondHalfLength];
			System.arraycopy(list, list.length/2, secondHalf, 0, secondHalfLength);
			mergeSort(secondHalf);
			int[] temp=merge(firstHalf,secondHalf);
			System.arraycopy(temp, 0, list, 0, temp.length);
		}
	}
	public static int[] merge(int[] list1,int[] list2){
		int[] temp=new int[list1.length+list2.length];
		int current1=0;
		int current2=0;
		int current3=0;
		while(current1<list1.length&¤t2<list2.length){
			if(list1[current1]<list2[current2])
				temp[current3++]=list1[current1++];
			else
				temp[current3++]=list2[current2++];
		}
		while(current1<list1.length)
			temp[current3++]=list1[current1++];
		while(current2<list2.length)
			temp[current3++]=list2[current2++];
		return temp;
				
	}
	public static void main(String[] args) {
		int[] list={2,3,2,5,6,1,-2,3,14,12};
		mergeSort(list);
		for(int i=0;i<list.length;i++)
			System.out.print(list[i]+"  ");
	}

}

归并算法的时间复杂度为O(nlogn)，优于冒泡、选择、插入，java.util.Arrays类中的sort方法就是使用归并排序算法的变体来实现的。