归并排序

归并排序的实现．时间复杂度是Ｏ(nlgn)，空间复杂度是O(n) + O(lgn)．

#include <iostream>

using namespace std;

void merge_array(int *, int, int, int);
void merge_sort(int *, int, int);

int main(void)
{

    int A[6] = {5,1,2,6,9,3};
    merge_sort(A, 0, 5);

    for (int i=0; i<6; ++i)
    {
        cout << A[i] << " ";
    }

    cout << endl;
    return 0;
}

void merge_sort(int A[], int p, int r)
{
    if (p < r)
    {
        int mid = (p + r) / 2;
        merge_sort(A, p, mid);
        merge_sort(A, mid+1, r);
        merge_array(A, p, mid, r);
    }
}

void merge_array(int A[], int p, int mid, int r)
{
    int *array1 = NULL;
    int *array2 = NULL;
    
    int array1_len = 0;
    int array2_len = 0;
    
    int i, j;

    array1_len = mid - p + 1;
    array2_len = r - mid;

    array1 = new int[array1_len];
    array2 = new int[array2_len];
    
    for (i=0; i<array1_len; ++i)
    {
        array1[i] = A[i+p];
    }

    for (j=0; j<array2_len; ++j)
    {
        array2[j] = A[mid+j+1];    
    }

    for (i=0, j=0; i<array1_len && j<array2_len;)
    {
        if (array1[i] < array2[j])
        {
            A[p++] = array1[i++];
        }
        else
        {
            A[p++] = array2[j++];
        }
    }

    while(i < array1_len)
    {
        A[p++] = array1[i++];
    }

    while(j < array2_len)
    {
        A[p++] = array2[j++];
    }
    
    delete [] array1;
    delete [] array2;
}