三种排序算法小结

首先是归并排序，基本思想为分治，合并的技巧比较重要，不是原址排序。

代码如下;

int merge(int* x,int left,int mid,int right)
{
    int i,j,k;
    int L1 = mid-left+2;
    int L2 = right-mid+1;
    int* L = new int[L1];
    int* R = new int[L2];
    memcpy(L,x+left,sizeof(int)*L1);
    memcpy(R,x+mid+1,sizeof(int)*L2);
    L[L1-1] = INT_MAX;
    R[L2-1] = INT_MAX;

    i = j = 0;
    k = left;
    while(k <= right)
    {
        if(L[i] > R[j])
        {
            x[k++] = R[j++];
        }
        else
        {
            x[k++] = L[i++];
        }
    }
    delete[] L;
    delete[] R;
    return 0;
}

int mergeSort(int* x,int left,int right)
{
    if(left == right)
        return 0;
    int mid = (left+right)>>1;
    mergeSort(x,left,mid);
    mergeSort(x,mid+1,right);
    merge(x,left,mid,right);
    return 0;
}

快速排序，应用最广的排序算法，把一个数组分成两段，一段的数据比一个数小，另一段则比它大，再往下分治。

代码如下;

int quickPartition(int* x,int left,int right)
{
    int k;
    srand((unsigned int)time(NULL));
    k = (rand()%(right-left+1))+left;
    swap(x[k],x[right]);
    k = left-1;
    for(int i = left;i < right;i++)
    {
        if(x[i] <= x[right])
        {
            swap(x[++k],x[i]);
        }
    }
    swap(x[++k],x[right]);
    return k;
}

int quickSort(int* x,int left,int right)
{
    if(left >= right)
    {
        return 0;
    }
    int mid = quickPartition(x,left,right);
    quickSort(x,left,mid-1);
    quickSort(x,mid+1,right);
    return 0;
}

最大堆排序算法，仅作为排序应用较少，更多地作为优先队列使用。

代码如下：

int maxHeapify(int* x,int i,int L)
{
    int large = i;
    int l = i<<1;
    int r = l+1;
    if(l <= L&&x[l] > x[large])
    {
        large = l;
    }
    if(r <= L&&x[r] > x[large])
    {
        large = r;
    }
    if(large != i)
    {
        swap(x[large],x[i]);
        maxHeapify(x,large,L);
    }
    return 0;
}

int buildMaxHeap(int* x,int L)
{
    for(int i = L>>1;i >= 1;i--)
    {
        maxHeapify(x,i,L);
    }
    return 0;
}

int heapSort(int* x,int L)
{
    buildMaxHeap(x,L);
    for(int i = L;i >= 2;i--)
    {
        swap(x[1],x[i]);
        maxHeapify(x,1,--L);
    }
    return 0;
}