归并排序

public class guiBing {
    private static void MergeSort(int[] a,int start,int end){
        if(start < end){
            //找分割位置
            int middle = (end + start) / 2;
            //递归分割下去,左边，右边
            MergeSort(a,start,middle);
            MergeSort(a,middle + 1,end);
            //该合并了
            merge(a,start,middle,end);
        }
    }

    public static void merge(int[] list,int start,int middle,int end){
        //leftLength和rightLength分别表示左边序列和右边序列的长度。左边从start开始包括middle，右边从middle+1开始
        int leftLength = middle - start + 1;
        int rightLength = end - middle;
        //定义俩个空的容器，将list分为左、右俩个序列，便于最后的排序
        int L[] = new int[leftLength];
        int R[] = new int[rightLength];
        //定义下标：L、R、list
        int lIndex = 0;
        int rIndex = 0;
        int listIndex = 0;

        //赋值 L 和 R
        for (lIndex = 0,listIndex = start; lIndex < leftLength; lIndex++,listIndex++) {
            L[lIndex] = list[listIndex];
        }

        for (rIndex = 0,listIndex = middle + 1; rIndex < rightLength; rIndex++,listIndex++) {
            R[rIndex] = list[listIndex];
        }

        //弟兄们，我要开始合体了，小的给我排前面去
        for(lIndex = 0,rIndex = 0,listIndex = start;
            lIndex < leftLength && rIndex < rightLength;
            listIndex++){
            //谁小谁先放进list中
            if(L[lIndex] <= R[rIndex]){
                list[listIndex] = L[lIndex];
                lIndex++;
            } else {
                list[listIndex] = R[rIndex];
                rIndex++;
            }
        }
        //将 L/R中 剩下的元素 放入 list，另外下面这俩个if，只会执行一个
        if(lIndex < leftLength){
            for (int tempIndex = lIndex; tempIndex < leftLength; tempIndex++,listIndex++) {
                list[listIndex] = L[tempIndex];
            }
        }

        if(rIndex < rightLength){
            for (int tempIndex = rIndex; tempIndex < rightLength; tempIndex++,listIndex++) {
                list[listIndex] = R[tempIndex];
            }
        }
    }

    public static void main(String[] args) {
        int i=0;
        int a[] = {5,4,9,8,7,6,0,1,3,2};
        int len = a.length;
        MergeSort(a,0,len-1);
        for(i=0; i<len; i++){
            System.err.print(a[i]+" ");
        }
    }
}