每周一算法之四——归并排序

归并排序，就是把两个有序序列合并成一个新的有序序列的算法。

也不复杂，用两个指针从前往后遍历两个序列，每次把小的那个元素取出来放到新的序列当中，取完了就Ok啦。

纠结的是临界条件的判断，得想想明白，哈哈。

// 归并排序
// C++实现
#include <iostream>

using namespace std;

template<class T>
void merge(T array[], int low, int high, int middle)
{
    int begin1 = low;
    int end1 = middle;
    int begin2 = middle+1;
    int end2 = high;
    int count = 0;

    T *buffer = new T[high-low+1];
    
    // 每次取两个序列中较小的值存放
    while( begin1 <= end1 && begin2 <= end2 )
    {
        if(array[begin1] <= array[begin2])
        {
            buffer[count++] = array[begin1++];
        }
        else
        {
            buffer[count++] = array[begin2++];
        }
    }
    
    // 复制序列余下的部分
    if( begin1 <= end1 )
    {
        memcpy(buffer+count, array+begin1, (end1-begin1+1)*sizeof(T));
    }
    if( begin2 <= end2 )
    {
        memcpy(buffer+count, array+begin2, (end2-begin2+1)*sizeof(T));
    }

    memcpy(array+low, buffer, (high-low+1)*sizeof(T));
    delete []buffer;
}

template<class T>
void merge_sort(T array[], int low, int high)
{
    if(low < high)
    {
        int middle = low/2 + high/2;
        merge_sort(array, low, middle);
        merge_sort(array, middle+1, high);
        merge(array, low, high, middle);
    }
}

template<class T>
void merge_sort(T array[], const int size)
{
    merge_sort(array, 0, size);
}

int main(void)
{
    int array[] = {2, 5, 9, 3, 12, 1, 6, 11, 4, 7, 13, 8, 10};
    merge_sort(array, sizeof(array) / sizeof(int));

    for (int i = 0; i < sizeof(array) / sizeof(int); i ++)
        cout<<array[i]<<endl;
            
    return 0;
}