它可以正确地比较对象,但是使用重复对象将列表的大小加倍。
public static void mergeSort(List<Person> list) {
List<Person> helper = new ArrayList<Person>(list.size());
mergeSort(list, helper, 0, list.size());
}
private static void mergeSort(List<Person> list, List<Person> helper, int low, int high) {
if(low < high) {
int middle = (low+high)/2;
mergeSort(list, helper, low, middle); //sort left half
mergeSort(list, helper, middle+1, high); //sort right half
merge(list, helper, low, middle, high); // merge
}
}
private static void merge(List<Person> list, List<Person> helper, int low, int middle, int high) {
for(int i=low; i<= high; i++) {
helper.add(i, list.get(i));
}
int helperLeft = low;
int helperRight = middle + 1;
int current = low;
/**
* Iterate through helper array, copying back smaller element in the original list
*/
while(helperLeft <= middle && helperRight <= high) {
if(helper.get(helperLeft).isLessThan( helper.get(helperRight))) {
list.add(current, helper.get(helperLeft));
helperLeft++;
} else {
list.add(current, helper.get(helperRight));
helperRight++;
}
current++;
}
//Copy remaining elements
int remaining = middle - helperLeft;
for(int j=0; j <= remaining; j++) {
list.add(current+j, helper.get(helperLeft+j));
}
}
Person.java
public class Person implements Comparable<Person>{
private String personId;
private String month;
private String day;
private String year;
private Date personDay;
static SimpleDateFormat formatter = new SimpleDateFormat("MM/dd/yyyy");
public Person(String id, String month, String day, String year) {
this.personId = id;
this.month = month;
this.day = day;
this.year = year;
}
@Override
public int compareTo(Person person) {
return this.getPersonDay().compareTo(person.getPersonDay());
}
public boolean isLessThan(Person person) {
boolean isLess = false;
if(this.getPersonDay().compareTo(person.getPersonDay()) < 0) {
isLess = true;
}
return isLess;
}
}
测试对象
List<Person> list = new ArrayList<Person>();
list.add(new Person("1L", "10", "1", "1960"));
list.add(new Person("1L", "4", "5", "1978"));
list.add(new Person("1L", "9", "17", "1986"));
list.add(new Person("1L", "2", "15", "1971"));
list.add(new Person("1L", "7", "1", "1971"));
答案 0 :(得分:3)
有几个问题......
请勿使用List#add
,请使用List#set
而不是......
while(helperLeft <= middle && helperRight <= high) {
if(helper.get(helperLeft).isLessThan( helper.get(helperRight))) {
list.add(current, helper.get(helperLeft));
helperLeft++;
} else {
list.add(current, helper.get(helperRight));
helperRight++;
}
current++;
}
//Copy remaining elements
int remaining = middle - helperLeft;
for(int j=0; j <= remaining; j++) {
list.add(current+j, helper.get(helperLeft+j));
}
使用...
while(helperLeft <= middle && helperRight <= high) {
if (helper.get(helperLeft).isLessThan(helper.get(helperRight))) {
list.set(current, helper.get(helperLeft));
helperLeft++;
} else {
list.set(current, helper.get(helperRight));
helperRight++;
}
current++;
}
//Copy remaining elements
int remaining = middle - helperLeft;
for (int j = 0; j <= remaining; j++) {
list.set(current + j, helper.get(helperLeft + j));
}
List
和数组一样,是零索引的,所以不是......
for (int i = low; i <= high; i++) {
helper.add(i, list.get(i));
}
你应该使用
for (int i = low; i < high; i++) {
helper.add(i, list.get(i));
}
而不是......
while (helperLeft <= middle && helperRight <= high) {
你应该使用......
while (helperLeft < middle && helperRight < high) {
因此,在进行了更正以适应示例中的缺失代码(例如getPersonDay
)后,我得到类似的内容......
----Unsorted
1L; 10/01/1960
1L; 04/05/1978
1L; 09/17/1986
1L; 02/15/1971
1L; 07/01/1971
----Sorted
1L; 10/01/1960
1L; 02/15/1971
1L; 07/01/1971
1L; 04/05/1978
1L; 09/17/1986
答案 1 :(得分:0)
当高 - 低<2时,你忘了对List的这个小范围进行排序。你需要处理那个小案例,如下所示
private static void mergeSort(List<Person> list, List<Person> helper, int low, int high) {
if(high - low<2) {
sortTheSmallList(list,low,high);
}else{
int middle = (low+high)/2;
mergeSort(list, helper , low, middle); //sort left half
mergeSort(list, helper, middle+1, high); //sort right half
merge(list, helper, low, middle, high);
}
}