归并排序

算法思想：

算法将数组分为两个子数组，然后对每个子数组递归应用归并排序。

package introjava;

public class MergeSort {
    public static void mergeSort(int [] list){
        if(list.length > 1){ //这是递归跳出的条件，一定要记住！！！
            int firstHalfLen = list.length / 2;
            int [] firstHalf = new int[firstHalfLen];
            System.arraycopy(list, 0, firstHalf, 0, firstHalfLen);
            mergeSort(firstHalf);

            int []secondHalf = new int [list.length - firstHalfLen];
            System.arraycopy(list, firstHalfLen, secondHalf, 0, list.length - firstHalfLen);　　
            mergeSort(secondHalf);

            int [] temp = merge(firstHalf, secondHalf);
            System.arraycopy(temp, 0, list, 0, list.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 && current2 < 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] + " ");
    }
}