36.数组中的逆序对

int InversePairs(int* data, int length)

{

if (data == NULL || length < 0)

return 0;

int *copy = new int[length];

for (int i = 0; i < length; ++i)

copy[i] = data[i];

int count = InversePairsCore(data, copy, 0, length - 1);

delete[] copy;

return count;

}

int InversePairsCore(int* data, int* copy, int start, int end)

{

if (start == end)

{

copy[start] = data[start];

return 0;

}

int length = (end - start) / 2;

int left = InversePairsCore(copy, data, start, start + length);

int right = InversePairsCore(copy, data, start + length + 1, end);

//i初始化为前半段最后一个数字的下标

int i = start + length;

//j初始化为后半段最后一个数字的下标

int j = end;

int indexCopy = end;

int count = 0;

while (i >= start&& j >= start + length + 1)

{

if (data[i] > data[j])

{

copy[indexCopy--] = data[i--];

count += j - start - length;

}

else

{

copy[indexCopy--] = data[j--];

}

}

for (; i >= start; --i)

copy[indexCopy--] = data[i];

for (; j >= start + length + 1; --j)

copy[indexCopy--] = data[j];

return left + right + count;

}