P1177 【模板】快速排序

最新推荐文章于 2024-03-04 14:15:00 发布

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最新推荐文章于 2024-03-04 14:15:00 发布

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本文深入探讨了快速排序算法的优化方案，对比了不同key值选取方式对算法效率的影响，并通过具体代码实现展示了优化前后的效果。文章提供了详细的C++代码示例，包括超时的原始方案和优化后的方案，适合对算法优化感兴趣的读者。

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https://www.luogu.org/problem/P1177

#include <iostream>
#include <fstream>
using namespace std;

int n;
int inputList[100000];
bool test = false;

void swap(int i, int j)
{
    if(test)
    {
        cout << "i=" << i << ",j=" << j << endl;
    }
    if (i == j)
    {
        return;
    }

    int tmp = inputList[i];
    inputList[i] = inputList[j];
    inputList[j] = tmp;
    //cout << i << "," << j << "-";
    //for (int t = 0; t < n; t++)
    //{
    //    cout << inputList[t] << " ";
    //}
    //cout <<endl;
}

// 备注：此方案会超时，应该是key的取值不合适
// [start, end)
void quickSort(int start, int end)
{
    if (start+1 >= end)
    {
        return;
    }
    int key = inputList[start];
    int i = start;
    int j = end - 1;
    bool bFront = false;
    while ( i != j)
    {
        if (bFront)
        {
            if (inputList[i] >= key)
            {
                //cout << start << "," << end << " , ";
                swap(i, j);
                bFront = false;
                j--;
            }
            else
            {
                i++;
            }
        } 
        else
        {
            if (inputList[j] < key)
            {
                //cout << start << "," << end << " , ";
                swap(i, j);
                bFront = true;
                i++;
            }
            else
            {
                j--;
            }
        }
    }
    quickSort(start, i);
    quickSort(i+1, end);
}

// 优化后方案，key取中间值
// [start, end]
void quickSort2(int start, int end)
{
    //if (start+1 >= end)
    //{
    //    return;
    //}
    int key = inputList[(start+end)/2];

    if(test)
    {
        cout << "[" << start << "," << end << "] key=" << key << endl;
    }

    int i = start;
    int j = end;
    while ( i <= j)
    {
        while(i <= end && inputList[i] < key)
        {
            i++;
        }
        //while(j >= start && inputList[j] >= key)
        while(j >= start && inputList[j] > key)
        {
            j--;
        }
        if (i <= j)
        {
            swap(i, j);
            i++;
            j--;
        }
    }
    if(i < end && i > start)
    {
        quickSort2(i, end);
    }
    if (j > start && j < end)
    {
        quickSort2(start, j);
    }
}

int main()
{
    //test = true;
    if (test)
    {
        ifstream f("C:\\Users\\suyang\\Downloads\\testdata (5).in");
        if (f)
        {
            cout << "f is ok" << endl;
        }
        else
        {
            cout << "f is error" << endl;
        }
        f >> n;
        for(int i = 0; i < n; i++)
        {
            f>>inputList[i];
        }
        f.close();
        n = 50;
    } 
    else
    {
        cin >> n;
        for(int i = 0; i < n; i++)
        {
            cin>>inputList[i];
        }
    }


    quickSort2(0, n-1);
    //quickSort(0, n);

    if (test)
    {
        ofstream f("luogu_out.txt");
        if (f)
        {
            cout << "f is ok" << endl;
        }
        else
        {
            cout << "f is error" << endl;
        }
        //f << inputList[0];
        //for(int i = 1; i < n; i++)
        //{
        //    f << " " << inputList[i];
        //}
        //f << endl;
        for(int i = 0; i < n; i++)
        {
            f << inputList[i] << endl;
        }
        f.close();
    }
    else
    {
        cout << inputList[0];
        for(int i = 1; i < n; i++)
        {
            cout << " " << inputList[i];
        }
        cout << endl;
    }
    return 0;
}