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Commit d4a020a

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merge sort
1 parent f4319cf commit d4a020a

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2 files changed

+144
-174
lines changed

2 files changed

+144
-174
lines changed

‎src/main/java/grey/algorithm/Code_0014_MergeSort.java

Lines changed: 129 additions & 174 deletions
Original file line numberDiff line numberDiff line change
@@ -16,179 +16,134 @@
1616

1717
public class Code_0014_MergeSort {
1818

19-
// merge sort的递归写法
20-
public static void sort(int[] arr) {
21-
mergeSort(arr, 0, arr.length - 1);
22-
}
23-
24-
// 递归解法
25-
public static void mergeSort(int[] arr, int l, int r) {
26-
if (l >= r) {
27-
// 终止条件
28-
return;
29-
}
30-
int mid = l + ((r - l) >> 1);
31-
mergeSort(arr, l, mid);
32-
mergeSort(arr, mid + 1, r);
33-
merge2(arr, l, mid, r);
34-
}
35-
36-
public static void merge2(int[] arr, int l, int m, int r) {
37-
int[] help = new int[r - l + 1];
38-
int s = l;
39-
int e = m + 1;
40-
int i = 0;
41-
while (s <= m && e <= r) {
42-
if (arr[s] <= arr[e]) {
43-
help[i++] = arr[s++];
44-
} else {
45-
help[i++] = arr[e++];
46-
}
47-
}
48-
while (s <= m) {
49-
help[i++] = arr[s++];
50-
}
51-
while (e <= r) {
52-
help[i++] = arr[e++];
53-
}
54-
for (i = 0; i < help.length; i++) {
55-
arr[l++] = help[i];
56-
}
57-
}
58-
59-
// 迭代版本的merge sort
60-
public static void mergeSort2(int[] arr, int l, int r) {
61-
if (arr == null || arr.length < 2) {
62-
return;
63-
}
64-
65-
int n = arr.length;
66-
67-
// 1. 外层循环:控制每次要合并的子数组的大小(subArraySize)
68-
// subArraySize 从 1 开始,每次翻倍,表示当前有序子数组的长度
69-
// 依次为 1, 2, 4, 8, ...
70-
for (int subArraySize = 1; subArraySize < n; subArraySize = subArraySize * 2) {
71-
72-
// 2. 内层循环:从左到右,找到每一对需要合并的子数组,然后调用merge
73-
// leftStart 是每一对子数组中,左边那个数组的起始位置
74-
for (int leftStart = 0; leftStart < n; leftStart = leftStart + subArraySize * 2) {
75-
76-
// 确定左子数组的边界
77-
// 左数组是 arr[leftStart ... mid]
78-
int mid = leftStart + subArraySize - 1;
79-
80-
// 如果 mid 已经越界,说明在 leftStart 之后连一个完整的左数组都凑不齐了
81-
// 那么剩下的部分自然就是有序的,无需再合并,直接跳出内层循环
82-
if (mid >= n - 1) {
83-
break;
84-
}
85-
86-
// 确定右子数组的边界
87-
// 右数组是 arr[mid + 1 ... rightEnd]
88-
// 这里用 Math.min 是为了防止右边界越界
89-
// 比如数组长度为13,当合并size=4的子数组时,最后一组是 arr[8...11] 和 arr[12...12]
90-
// 此时 rightEnd 就不能是 8 + 4*2 - 1 = 15,而应该是 n - 1 = 12
91-
int rightEnd = Math.min(leftStart + subArraySize * 2 - 1, n - 1);
92-
93-
// 找到了左右两个要合并的有序子数组,执行合并
94-
// System.out.printf("合并: subArraySize=%d, merge(arr, %d, %d, %d)%n", subArraySize, leftStart, mid, rightEnd);
95-
merge(arr, leftStart, mid, rightEnd);
96-
}
97-
// System.out.println("--- subArraySize=" + subArraySize + " 的一轮合并结束 ---");
98-
}
99-
}
100-
101-
private static void merge(int[] arr, int l, int m, int r) {
102-
int i = 0;
103-
int s = l;
104-
int e = m + 1;
105-
int[] help = new int[r - l + 1];
106-
while (s <= m && e <= r) {
107-
if (arr[s] <= arr[e]) {
108-
help[i++] = arr[s++];
109-
} else {
110-
help[i++] = arr[e++];
111-
}
112-
}
113-
while (s <= m) {
114-
help[i++] = arr[s++];
115-
}
116-
117-
while (e <= r) {
118-
help[i++] = arr[e++];
119-
}
120-
for (i = 0; i < help.length; i++) {
121-
arr[l++] = help[i];
122-
}
123-
}
124-
125-
public static void swap(int[] arr, int i, int j) {
126-
if (null == arr || arr.length <= 1) {
127-
return;
128-
}
129-
int tmp = arr[i];
130-
arr[i] = arr[j];
131-
arr[j] = tmp;
132-
}
133-
134-
public static void main(String[] args) {
135-
136-
// 数组长度1~500,等概率随机
137-
int num = 500;
138-
// 每个值的大小在1~1024,等概率随机
139-
int value = 1024;
140-
// 测试次数
141-
int testTimes = 50000;
142-
System.out.println("测试开始");
143-
for (int i = 0; i < testTimes; i++) {
144-
int[] arr = generateArray(num, value);
145-
int[] copyArray1 = copyArray(arr);
146-
int[] copyArray2 = copyArray(arr);
147-
Arrays.sort(arr);
148-
sort(copyArray1);
149-
mergeSort2(copyArray2, 0, copyArray2.length - 1);
150-
if (!sameValue(arr, copyArray1)) {
151-
System.out.println("出错了!");
152-
break;
153-
}
154-
if (!sameValue(arr, copyArray2)) {
155-
System.out.println("出错了!");
156-
break;
157-
}
158-
}
159-
System.out.println("测试结束");
160-
}
161-
162-
private static boolean sameValue(int[] arr1, int[] arr2) {
163-
if (null == arr1) {
164-
return null != arr2;
165-
}
166-
if (null == arr2) {
167-
return null != arr1;
168-
}
169-
if (arr1.length != arr2.length) {
170-
return false;
171-
}
172-
for (int i = 0; i < arr1.length; i++) {
173-
if (arr1[i] != arr2[i]) {
174-
return false;
175-
}
176-
}
177-
return true;
178-
}
179-
180-
private static int[] generateArray(int num, int value) {
181-
int[] arr = new int[(int) (Math.random() * num) + 1];
182-
for (int i = 0; i < arr.length; i++) {
183-
arr[i] = (int) (Math.random() * value) + 1;
184-
}
185-
return arr;
186-
}
187-
188-
private static int[] copyArray(int[] arr) {
189-
int[] copyArray = new int[arr.length];
190-
System.arraycopy(arr, 0, copyArray, 0, copyArray.length);
191-
return copyArray;
192-
}
19+
// merge sort的递归写法
20+
public static void sort(int[] arr) {
21+
mergeSort(arr, 0, arr.length - 1);
22+
}
23+
24+
// 递归解法
25+
public static void mergeSort(int[] arr, int l, int r) {
26+
if (l >= r) {
27+
// 终止条件
28+
return;
29+
}
30+
int mid = l + ((r - l) >> 1);
31+
mergeSort(arr, l, mid);
32+
mergeSort(arr, mid + 1, r);
33+
merge(arr, l, mid, r);
34+
}
35+
36+
public static void merge(int[] arr, int l, int m, int r) {
37+
int[] help = new int[r - l + 1];
38+
int s = l;
39+
int e = m + 1;
40+
int i = 0;
41+
while (s <= m && e <= r) {
42+
if (arr[s] <= arr[e]) {
43+
help[i++] = arr[s++];
44+
} else {
45+
help[i++] = arr[e++];
46+
}
47+
}
48+
while (s <= m) {
49+
help[i++] = arr[s++];
50+
}
51+
while (e <= r) {
52+
help[i++] = arr[e++];
53+
}
54+
for (i = 0; i < help.length; i++) {
55+
arr[l++] = help[i];
56+
}
57+
}
58+
59+
// 迭代版本的merge sort
60+
public static void sort2(int[] arr) {
61+
if (arr == null || arr.length < 2) {
62+
return;
63+
}
64+
int n = arr.length;
65+
for (int subSize = 1; subSize < n; subSize <<= 1) {
66+
// 每组1,2,4,8这样扩
67+
for (int leftIndex = 0; leftIndex < n; leftIndex = leftIndex + (subSize * 2)) {
68+
// 第一组的起始位置
69+
int m = leftIndex + subSize - 1;
70+
if (m >= n - 1) {
71+
// 没有右组,已经排好了
72+
break;
73+
}
74+
int rightEnd = Math.min(leftIndex + (subSize * 2) - 1, n - 1);
75+
merge(arr, leftIndex, m, rightEnd);
76+
}
77+
}
78+
}
79+
80+
public static void swap(int[] arr, int i, int j) {
81+
if (null == arr || arr.length <= 1) {
82+
return;
83+
}
84+
int tmp = arr[i];
85+
arr[i] = arr[j];
86+
arr[j] = tmp;
87+
}
88+
89+
public static void main(String[] args) {
90+
91+
// 数组长度1~500,等概率随机
92+
int num = 500;
93+
// 每个值的大小在1~1024,等概率随机
94+
int value = 1024;
95+
// 测试次数
96+
int testTimes = 50000;
97+
System.out.println("测试开始");
98+
for (int i = 0; i < testTimes; i++) {
99+
int[] arr = generateArray(num, value);
100+
int[] copyArray1 = copyArray(arr);
101+
int[] copyArray2 = copyArray(arr);
102+
Arrays.sort(arr);
103+
sort(copyArray1);
104+
sort2(copyArray2);
105+
if (!sameValue(arr, copyArray1)) {
106+
System.out.println("出错了!");
107+
break;
108+
}
109+
if (!sameValue(arr, copyArray2)) {
110+
System.out.println("出错了!");
111+
break;
112+
}
113+
}
114+
System.out.println("测试结束");
115+
}
116+
117+
private static boolean sameValue(int[] arr1, int[] arr2) {
118+
if (null == arr1) {
119+
return null != arr2;
120+
}
121+
if (null == arr2) {
122+
return null != arr1;
123+
}
124+
if (arr1.length != arr2.length) {
125+
return false;
126+
}
127+
for (int i = 0; i < arr1.length; i++) {
128+
if (arr1[i] != arr2[i]) {
129+
return false;
130+
}
131+
}
132+
return true;
133+
}
134+
135+
private static int[] generateArray(int num, int value) {
136+
int[] arr = new int[(int) (Math.random() * num) + 1];
137+
for (int i = 0; i < arr.length; i++) {
138+
arr[i] = (int) (Math.random() * value) + 1;
139+
}
140+
return arr;
141+
}
142+
143+
private static int[] copyArray(int[] arr) {
144+
int[] copyArray = new int[arr.length];
145+
System.arraycopy(arr, 0, copyArray, 0, copyArray.length);
146+
return copyArray;
147+
}
193148

194149
}

‎src/main/java/grey/algorithm/Main.java

Lines changed: 15 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,15 @@
1+
/*
2+
* Click nbfs://nbhost/SystemFileSystem/Templates/Licenses/license-default.txt to change this license
3+
* Click nbfs://nbhost/SystemFileSystem/Templates/Classes/Class.java to edit this template
4+
*/
5+
package grey.algorithm;
6+
7+
/**
8+
*
9+
* @author Grey
10+
*/
11+
public class Main {
12+
public static void main(String[] arr) {
13+
// 用作测试
14+
}
15+
}

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