归并排序求数组逆序对数

#include <iostream>

using namespace std;

const int LENGTH = 100;
int temp[LENGTH];
int count = 0;

void Merge(int array[], int low, int mid, int high);
void MergeSort(int array[], int low, int high);

/**
 * 合并两个数组[low, mid],(mid, high]到数组[low, high]
 * @param array 待排序数组
 * @param low   起始下标
 * @param mid   分界线
 * @param high  终止下标
 */
void Merge(int array[], int low, int mid, int high)
{
    int tempIndex = low;
    int leftIndex = low;
    int rightIndex = mid + 1;

    while(leftIndex <= mid && rightIndex <= high)
    {
        if(array[leftIndex] < array[rightIndex])
        {
            temp[tempIndex++] = array[leftIndex++];
        }
        else // array[rightIndex]要比[leftIndex, mid]的元素都小，逆序对由此产生
        {
            temp[tempIndex++] = array[rightIndex++];
            count += (mid - leftIndex + 1);
        }
    }

    while(leftIndex <= mid)
    {
        temp[tempIndex++] = array[leftIndex++];
    }

    while(rightIndex <= high)
    {
        temp[tempIndex++] = array[rightIndex++];
    }

    for(int i = low; i <= high; ++i)
    {
        array[i] = temp[i];
    }
}

void MergeSort(int array[], int low, int high)
{
    if(low == high) return;

    int mid = (low + high) / 2;
    MergeSort(array, low, mid);
    MergeSort(array, mid + 1, high);
    Merge(array, low, mid, high);
}

int main()
{
    int array[] = {0, 4, 2, 6, 12, 10, 8, 7, 5};
    MergeSort(array, 0, 8);

    for(auto val : array)
    {
        cout << val << " ";
    }
    cout << endl;
    cout << "--------------------------" << endl;
    cout << count << endl;

    return 0;
}

说明：需要编译器支持C++11

PS C:\Users\Administrator\Desktop\codes> g++ -o .\InverseNumber.exe .\InverseNumber.cpp -std=c++11
PS C:\Users\Administrator\Desktop\codes> .\InverseNumber.exe
0 2 4 5 6 7 8 10 12
--------------------------
12
PS C:\Users\Administrator\Desktop\codes> g++ -v
Using built-in specs.
COLLECT_GCC=F:\GreenSoftware\CodeBlocks\TDM-GCC\bin\g++.exe
COLLECT_LTO_WRAPPER=F:/GreenSoftware/CodeBlocks/TDM-GCC/bin/../libexec/gcc/mingw32/5.1.0/lto-wrapper.exe
Target: mingw32
Configured with: ../../../src/gcc-5.1.0/configure --build=mingw32 --enable-languages=ada,c,c++,fortran,lto,objc,obj-c++
--enable-libgomp --enable-lto --enable-graphite --enable-libstdcxx-debug --enable-threads=posix --enable-version-specifi
c-runtime-libs --enable-fully-dynamic-string --enable-libstdcxx-threads --enable-libstdcxx-time --with-gnu-ld --disable-
werror --disable-nls --disable-win32-registry --disable-symvers --enable-cxx-flags=‘-fno-function-sections -fno-data-sec
tions -DWINPTHREAD_STATIC‘ --prefix=/mingw32tdm --with-local-prefix=/mingw32tdm --with-pkgversion=tdm-1 --enable-sjlj-ex
ceptions --with-bugurl=http://tdm-gcc.tdragon.net/bugs
Thread model: posix
gcc version 5.1.0 (tdm-1)
PS C:\Users\Administrator\Desktop\codes>