排序算法·合并排序

算法讲解请参阅下面参考书籍，这里只给出自己练习时的代码实现。
参考书籍：《算法设计与分析基础·3ed》

算法1:
Mergesort(A[0..n-1])
//递归调用mergesort来对数组A[0..n-1]排序
//输入：一个可排序数组A[0..n-1]
//输出：非降序排列的数组A[0..n-1]
if n > 1
copy A[0..⌊n/2⌋-1] to B[0..⌊n/2⌋-1]
copy A[⌊n/2⌋..n-1] to C[0..⌈n/2⌉-1]
Mergesort(B[0..⌊n/2⌋-1])
Mergesort(C[0..⌈n/2⌉-1])
Merge(B,C,A)

算法2:
Merge(B[0..p-1], C[0..q-1], A[0..p+q-1])
//将两个有序数组合并为一个有序数组
//输入：两个有序数组B[0..p-1]和C[0..q-1]
//输出：A[0..p+q-1]中已经有序存放了B和C中的元素
i = 0; j = 0; k = 0;
while i<p and j<q do
if B[i] <= C[j]
A[k] = B[i];i = i+1;
else
A[k] = C[j];j = j+1;
k = k+1
if i = p
copy C[j..q-1] to A[k..p+q-1]
else
copy B[i..p-1] to A[k..p+q-1]

#include <iostream>

using namespace std;

void merge_sort(int arr[], int length);
void merge_arr(int arr_1[], int arr_2[], int arr[], int p, int q);

int main()
{
int arr[] = {25, 45, 15, 40, 90, 5, 85, 75, 32, 65, 10, 5, 100, 15};
int length = sizeof(arr) / sizeof(int);

merge_sort(arr, length);

for (int i = 0; i < length; i++)
cout << arr[i] << " ";
cout << endl;

return 0;
}

//Mergesort
void merge_sort(int arr[], int length)
{
if(length > 1){
int length_1 = length/2, length_2;

if(length % 2 == 0)
length_2 = length_1;
else
length_2 = length_1 + 1;

int arr_1[length_1];
int arr_2[length_2];

for(int i = 0; i < length_1; i++)
arr_1[i] = arr[i];
for(int i = 0; i < length_2; i++)
arr_2[i] = arr[length/2 + i];

merge_sort(arr_1, length_1);
merge_sort(arr_2, length_2);
merge_arr(arr_1, arr_2, arr, length_1, length_2);
}
}

//Merge
void merge_arr(int arr_1[], int arr_2[], int arr[], int p, int q)
{
int i = 0, j = 0, k = 0;

while(i < p && j < q){
if(arr_1[i] <= arr_2[j]){
arr[k] = arr_1[i];
i++;
}else{
arr[k] = arr_2[j];
j++;
}
k++;
}

if(i == p)
for(int t = j; t <= q-1; t++, k++)
arr[k] = arr_2[t];
else
for(int t = i; t <= p-1; t++, k++)
arr[k] = arr_1[t];
}