归并排序也是排序算法的一种,它是将两个已经排好序的数组,合并成另一个排好序的数组,原理是这样的:定义一个新数组,再定义两个指针,分别指向两个已经排好序的数组的第一个元素,然后两者进行比较,较小的那个数放到新定义的那个数组的第一个位置,同时,将较小的那个数的下标加1,再跟刚刚比他大的那个数比较,两者中较小的数就放到新数组里面,最后比较完之后,若其中一个数组有剩余的元素没有比较,就将剩余的元素直接插入到新数组中。
例如:
代码体现:
package edu.xalead;
public class 合并排序 {
public static void mergeSort(int[] a, int left, int right) {
if (left < right) {
//至少有两个元素
int i = (left + right) / 2; //取中点
mergeSort(a, left, i);
mergeSort(a, i + 1, right);
Comparable[] b = null;
merge(a,b,left,i,right); // 合并到数组b
copy(a,b,left,right); // 复制回数组a
}
}
public static void copy(Comparable[] a, Comparable[] b, int left, int right) {
for (int i = left; i < right; i++) {
a[i] = b[i];
}
}
// public static void mergeSort(Comparable[] a) {
////
//// Comparable[] b = new Comparable[a.length];
//// int s = 1;
//// while(s<a.length){
//// mergePass(a,b,s); //合并到数组b;
//// s+=s;
//// mergePass(b,a,s); //合并到数组a;
//// s+=s;
//// }
////
//// }
public static void mergePass(Comparable[] x, Comparable[] y, int s) {
//合并排好序的相邻子数组
int i = 0;
while (i <= x.length - 2 * s) {
merge(x, y, i, i + s - 1, i + 2 * s - 1);
i = i + 2 * s;
}
if (i + s < x.length)
merge(x, y, i, i + s - 1, x.length - 1);
else {
for (int j = i; j < x.length; j++) {
y[j] = x[j];
}
}
}
public static void merge(Comparable[] c, Comparable[] d, int l, int m, int r) {
int i = l,
j = m + 1,
k = l;
while ((i <= m) && (j <= r)) {
if (c[i].compareTo(c[j]) <= 0)
d[k++] = c[i++];
else d[k++] = c[j++];
if (i > m)
for (int q = j; q <= r; q++)
d[k++] = c[q];
else
for (int q = i; q <= m; q++)
d[k++] = c[q];
}
}
}
package edu.xalead;
import java.util.Arrays;
public class 合并2 {
public static void main(String[] args) {
int a[] = new int[] { 6, 10, 25, 3, 33, 90, 60, 100, 56 };
mergeSort(a);
print(a);
}
private static void mergeSort(int[] a) {
if (a== null) {
throw new NullPointerException("The array can not be null !!!");
}
int length = a.length;
if (length > 1) {
int middle = length / 2;
int partitionA[] = Arrays.copyOfRange(a, 0, middle);// 拆分问题规模
int partitionB[] = Arrays.copyOfRange(a, middle, length);
// 递归调用
mergeSort(partitionA);
mergeSort(partitionB);
sort(partitionA, partitionB,a);
}
}
private static void sort(int[] partitionA, int[] partitionB, int[] a) {
int i = 0;
int j = 0;
int k = 0;
//左右数据合并并入新数组
while (i < partitionA.length && j < partitionB.length) {
if (partitionA[i] <= partitionB[j]) {
a[k] = partitionA[i];
i++;
} else {
a[k] = partitionB[j];
j++;
}
k++;
}
// 取到中值时
if (i == partitionA.length) {
while (k < a.length) {
a[k] = partitionB[j];
k++;
j++;
}
} else if (j == partitionB.length) {
while (k < a.length) {
a[k] = partitionA[i];
k++;
i++;
}
}
}
private static void print(int[] array) {
if (array == null) {
throw new NullPointerException("The array can not be null !!!");
}
StringBuilder sb = new StringBuilder("[");
for (int element : array) {
sb.append(element + ", ");
}
sb.replace(sb.length() - 2, sb.length(), "]"); // 打印出数据后添加】
System.out.println(sb.toString());
}
}
运行结果: