剑指Offer34 数组中的逆序对

/*************************************************************************
    > File Name: 34_InversePairsInArray.c
    > Author: Juntaran
    > Mail: JuntaranMail@gmail.com
    > Created Time: 2016年09月02日 星期五 20时05分05秒
 ************************************************************************/

#include <stdio.h>
#include <malloc.h>

int InversePairsCore(int* nums, int* copy, int left, int right)
{
    if (left == right)
    {
        copy[left] = copy[right];
        return 0;
    }
    int newlength = (right - left) / 2;
    
    // 递归
    int leftCount  = InversePairsCore(copy, nums, left, left+newlength);
    int rightCount = InversePairsCore(copy, nums, left+newlength+1, right);
    
    // i初始化为前半段最后一个数字下标
    int i = left + newlength;
    // j初始化为后半段最后一个数字下标
    int j = right;
    
    int indexCopy = right;
    int count = 0;
    
    while (i>=left && j>=left+newlength+1)
    {
        if (nums[i] > nums[j])
        {
            copy[indexCopy--] = nums[i--];
            count += j - left - newlength;
        }
        else
        {
            copy[indexCopy--] = nums[j--];
        }
    }
    
    for (i; i >= left; --i)
        copy[indexCopy--] = nums[i];
    for (j; j>=left+newlength+1; --j)
        copy[indexCopy--] = nums[j];
    
    return leftCount + rightCount + count;
}

int InversePairs(int* nums, int length)
{
    if (nums==NULL || length<=0)
        return -1;
    
    int* copy = (int*)malloc(sizeof(int)*length);
    for (int i = 0; i < length; ++i)
        copy[i] = nums[i];
    
    int count = InversePairsCore(nums, copy, 0, length-1);
    delete[] copy;
    
    return count;
}

int main()
{
    int nums[] = {7,5,6,4};
    int length = 4;
    int ret = InversePairs(nums, length);
    printf("%d
", ret);
}