归并排序

MergeSort：把整个数组分解为单个元素，再向上Merge

Merge：合并两个已经有序的数组为一个有序数组

 

public class P2_3_1 {

    public static void main(String[] args) {
        int[] A = {3,41,52,26,38,57,9,49};
        int p = 0,q = A.length-1;
        MergeSort(A,p,q);
        for(int i = 0;i < A.length;i++) {
            System.out.println(A[i]);
        }

    }
    public static void MergeSort(int[] A,int p,int q){
            if(p < q) {
                int r = (p+q)/2;
                MergeSort(A,p,r);
                MergeSort(A,r+1,q);
                Merge(A,p,r,q);
            }        
    }
    public static void Merge(int[] A,int p,int r,int q) {
        int n1 = r-p+1;
        int n2 = q-r;
        int[] L = new int[n1+1];
        int[] R = new int[n2+1];
        for(int i = 0;i < n1;i++) {
            L[i] = A[p+i];
        }
        for(int j = 0;j < n2;j++) {
            R[j] = A[r+1+j];
        }
        L[n1] = 999;    //999表示无穷大，作为哨兵
        R[n2] = 999;
        int i = 0,j = 0;
        for(int k = p;k < q+1;k++) {
            if(L[i] < R[j]) {
                A[k] = L[i];
                i = i+1;
            }else {
                A[k] = R[j];
                j = j+1;
            }
        }    
    }
}