归并排序

#include<iostream>
#define MAX 5000003
#define SENTINEL 2000000000
using namespace std;

int L[MAX / 2 + 2], R[MAX / 2 + 2];            
int cnt;

void merge (int A[],  int left, int mid, int right)  //合并 right为数组长 
{
    int n1 = mid - left;
    int n2 = right - mid;
    for( int i = 0; i < n1; i++ )        L[i] = A[left + i];
    for( int i = 0; i < n2; i++ )        R[i] = A[mid + i];
    L[n1] = R[n2] = SENTINEL;                                 //分到最小时L、R中各有一个数，若进行比较合并，需比较两次，才能合并入A，故需要一个 很大的sentinel ； 
    int i = 0, j = 0;
    for(int k = left; k < right; k++)
    {
        cnt++;
        if( L[i] <= R[j])
            A[k] = L[i++];
        else 
            A[k] = R[j++];
    }
    
}

void mergeSort( int A[], int left, int right)
{
    if( left + 1 < right)
    {
        int mid = (left + right) / 2;
        mergeSort(A, left, mid);
        mergeSort(A, mid, right);
        merge(A, left, mid,right);
    }
}

int main()
{
    int A[MAX], n, i;
    
    cin >> n;
    for( i = 0; i < n; i++)        cin >> A[i];
    
    mergeSort(A, 0, n);
    
    for( i = 0;i < n;i++ )
    {
        if( i ) cout << " ";
        cout << A[i];    
    } 
    cout << endl;
    
    return 0;
}