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feat: add MergeSort
1 parent 29a5561 commit 68df26c

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4 files changed

+162
-18
lines changed

4 files changed

+162
-18
lines changed

‎basic/sort/BubbleSort.java

+9-2
Original file line numberDiff line numberDiff line change
@@ -1,12 +1,13 @@
1+
import java.util.Arrays;
2+
13
public class BubbleSort {
24
private static void swap(int[] nums, int i, int j) {
35
int t = nums[i];
46
nums[i] = nums[j];
57
nums[j] = t;
68
}
79

8-
public static void main(String[] args) {
9-
int[] nums = {1, 2, 7, 9, 5, 8};
10+
private static void bubbleSort(int[] nums) {
1011
boolean hasChange = true;
1112
for (int i = 0; i < nums.length - 1 && hasChange; ++i) {
1213
hasChange = false;
@@ -18,4 +19,10 @@ public static void main(String[] args) {
1819
}
1920
}
2021
}
22+
23+
public static void main(String[] args) {
24+
int[] nums = {1, 2, 7, 9, 5, 8};
25+
bubbleSort(nums);
26+
System.out.println(Arrays.toString(nums));
27+
}
2128
}

‎basic/sort/InsertionSort.java

+10-5
Original file line numberDiff line numberDiff line change
@@ -1,14 +1,19 @@
1-
public class InsertionSort {
2-
3-
public static void main(String[] args) {
4-
int[] nums = { 1, 2, 7, 9, 5, 8 };
1+
import java.util.Arrays;
52

3+
public class InsertionSort {
4+
private static void insertionSort(int[] nums) {
65
for (int i = 1, j, current; i < nums.length; ++i) {
76
current = nums[i];
8-
for (j = i - 1; j >=0 && nums[j] > current; --j) {
7+
for (j = i - 1; j >= 0 && nums[j] > current; --j) {
98
nums[j + 1] = nums[j];
109
}
1110
nums[j + 1] = current;
1211
}
1312
}
13+
14+
public static void main(String[] args) {
15+
int[] nums = { 1, 2, 7, 9, 5, 8 };
16+
insertionSort(nums);
17+
System.out.println(Arrays.toString(nums));
18+
}
1419
}

‎basic/sort/MergeSort.java

+44
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,44 @@
1+
import java.util.Arrays;
2+
3+
public class MergeSort {
4+
5+
private static void merge(int[] nums, int low, int mid, int high, int[] temp) {
6+
int i = low, j = mid + 1, k = low;
7+
while (k <= high) {
8+
if (i > mid) {
9+
temp[k++] = nums[j++];
10+
} else if (j > high) {
11+
temp[k++] = nums[i++];
12+
} else if (nums[i] <= nums[j]) {
13+
temp[k++] = nums[i++];
14+
} else {
15+
temp[k++] = nums[j++];
16+
}
17+
}
18+
19+
for (i = low; i <= high; ++i) {
20+
nums[i] = temp[i];
21+
}
22+
}
23+
24+
private static void mergeSort(int[] nums, int low, int high, int[] temp) {
25+
if (low >= high) {
26+
return;
27+
}
28+
int mid = low + ((high - low) >> 1);
29+
mergeSort(nums, low, mid, temp);
30+
mergeSort(nums, mid + 1, high, temp);
31+
merge(nums, low, mid, high, temp);
32+
}
33+
34+
private static void mergeSort(int[] nums) {
35+
int[] temp = new int[nums.length];
36+
mergeSort(nums, 0, nums.length - 1, temp);
37+
}
38+
39+
public static void main(String[] args) {
40+
int[] nums = {1, 2, 7, 4, 5, 3};
41+
mergeSort(nums);
42+
System.out.println(Arrays.toString(nums));
43+
}
44+
}

‎basic/sort/README.md

+99-11
Original file line numberDiff line numberDiff line change
@@ -7,12 +7,33 @@
77
### 代码示例
88

99
```java
10-
for (int i = 0; i < nums.length - 1; ++i) {
11-
for (int j = 0; j < nums.length - 1 - i; ++j) {
12-
if (nums[j] > nums[j + 1]) {
13-
swap(nums, j, j + 1);
10+
import java.util.Arrays;
11+
12+
public class BubbleSort {
13+
private static void swap(int[] nums, int i, int j) {
14+
int t = nums[i];
15+
nums[i] = nums[j];
16+
nums[j] = t;
17+
}
18+
19+
private static void bubbleSort(int[] nums) {
20+
boolean hasChange = true;
21+
for (int i = 0; i < nums.length - 1 && hasChange; ++i) {
22+
hasChange = false;
23+
for (int j = 0; j < nums.length - 1 - i; ++j) {
24+
if (nums[j] > nums[j + 1]) {
25+
swap(nums, j, j + 1);
26+
hasChange = true;
27+
}
28+
}
1429
}
1530
}
31+
32+
public static void main(String[] args) {
33+
int[] nums = {1, 2, 7, 9, 5, 8};
34+
bubbleSort(nums);
35+
System.out.println(Arrays.toString(nums));
36+
}
1637
}
1738
```
1839

@@ -37,17 +58,29 @@ for (int i = 0; i < nums.length - 1; ++i) {
3758
- 在冒泡排序中，经过每一轮的排序处理后，数组后端的数是排好序的。
3859
- 在插入排序中，经过每一轮的排序处理后，数组前端的数是拍好序的。
3960

40-
插入排序算法的基本思想是：不断将尚未排好序的数插入到已经排好序的部分。
61+
插入排序的算法思想是：不断将尚未排好序的数插入到已经排好序的部分。
4162

4263
### 代码示例
4364

4465
```java
45-
for (int i = 1, j, current; i < nums.length; ++i) {
46-
current = nums[i];
47-
for (j = i - 1; j >=0 && nums[j] > current; --j) {
48-
nums[j + 1] = nums[j];
66+
import java.util.Arrays;
67+
68+
public class InsertionSort {
69+
private static void insertionSort(int[] nums) {
70+
for (int i = 1, j, current; i < nums.length; ++i) {
71+
current = nums[i];
72+
for (j = i - 1; j >= 0 && nums[j] > current; --j) {
73+
nums[j + 1] = nums[j];
74+
}
75+
nums[j + 1] = current;
76+
}
77+
}
78+
79+
public static void main(String[] args) {
80+
int[] nums = { 1, 2, 7, 9, 5, 8 };
81+
insertionSort(nums);
82+
System.out.println(Arrays.toString(nums));
4983
}
50-
nums[j + 1] = current;
5184
}
5285
```
5386

@@ -61,4 +94,59 @@ for (int i = 1, j, current; i < nums.length; ++i) {
6194
2. 给定的数组按照逆序排列：需要进行 `n*(n-1)/2` 次比较，时间复杂度为 O(n^2)，这是最坏的情况。
6295
3. 给定的数组杂乱无章：在这种情况下，平均时间复杂度是 O(n^2)。
6396

64-
因此，时间复杂度是 O(n^2)，这也是一种稳定的排序算法。
97+
因此，时间复杂度是 O(n^2)，这也是一种稳定的排序算法。
98+
99+
## 归并排序
100+
101+
归并排序的核心思想是分治，把一个复杂问题拆分成若干个子问题来求解。
102+
103+
归并排序的算法思想是：把数组从中间划分为两个子数组，一直递归地把子数组划分成更小的数组，直到子数组里面只有一个元素的时候开始排序。排序的方法就是按照大小顺序合并两个元素。接着依次按照递归的顺序返回，不断合并排好序的数组，直到把整个数组排好序。
104+
105+
### 代码示例
106+
107+
```java
108+
import java.util.Arrays;
109+
110+
public class MergeSort {
111+
112+
private static void merge(int[] nums, int low, int mid, int high, int[] temp) {
113+
int i = low, j = mid + 1, k = low;
114+
while (k <= high) {
115+
if (i > mid) {
116+
temp[k++] = nums[j++];
117+
} else if (j > high) {
118+
temp[k++] = nums[i++];
119+
} else if (nums[i] <= nums[j]) {
120+
temp[k++] = nums[i++];
121+
} else {
122+
temp[k++] = nums[j++];
123+
}
124+
}
125+
126+
for (i = low; i <= high; ++i) {
127+
nums[i] = temp[i];
128+
}
129+
}
130+
131+
private static void mergeSort(int[] nums, int low, int high, int[] temp) {
132+
if (low >= high) {
133+
return;
134+
}
135+
int mid = low + ((high - low) >> 1);
136+
mergeSort(nums, low, mid, temp);
137+
mergeSort(nums, mid + 1, high, temp);
138+
merge(nums, low, mid, high, temp);
139+
}
140+
141+
private static void mergeSort(int[] nums) {
142+
int[] temp = new int[nums.length];
143+
mergeSort(nums, 0, nums.length - 1, temp);
144+
}
145+
146+
public static void main(String[] args) {
147+
int[] nums = {1, 2, 7, 4, 5, 3};
148+
mergeSort(nums);
149+
System.out.println(Arrays.toString(nums));
150+
}
151+
}
152+
```

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