快速排序

快速排序有很多优化版本，在此仅列出最基本的方法。参见链接

/* This function takes last element as pivot,

       places the pivot element at its correct

       position in sorted array, and places all

       smaller (smaller than pivot) to left of

       pivot and all greater elements to right

       of pivot */

    int partition(int arr[], int low, int high)

    {

        int pivot = arr[high]; 

        int i = (low-1); // index of smaller element

        for (int j=low; j<high; j++)

        {

            // If current element is smaller than the pivot

            if (arr[j] < pivot)

            {

                i++;

  

                // swap arr[i] and arr[j]

                int temp = arr[i];

                arr[i] = arr[j];

                arr[j] = temp;

            }

        }

  

        // swap arr[i+1] and arr[high] (or pivot)

        int temp = arr[i+1];

        arr[i+1] = arr[high];

        arr[high] = temp;

  

        return i+1;

    }

  

  

    /* The main function that implements QuickSort()

      arr[] --> Array to be sorted,

      low  --> Starting index,

      high  --> Ending index */

    void sort(int arr[], int low, int high)

    {

        if (low < high)

        {

            /* pi is partitioning index, arr[pi] is 

              now at right place */

            int pi = partition(arr, low, high);

  

            // Recursively sort elements before

            // partition and after partition

            sort(arr, low, pi-1);

            sort(arr, pi+1, high);

        }

    }