#方法1：暴力法 0（n**2）
def  InversePairs(data):
    count=0
    for i in range(len(data)):
        for j in range(i,len(data)):
            if data[i]>data[j]:
                count += 1          
    return count
data=[7,5,6,4]
InversePairs(data)

#方法2：利用归并排序思想0（nlogn）
def  InversePairs(data):
    import copy
    n= len(data)
    if not data  or n<0:
        return 0
    datacopy = copy.deepcopy(data)
    return InversePairsCore(data,datacopy,0,n-1)
def InversePairsCore(data,datacopy,start,end):
    if start == end:
        return 0
    mid=(start+end)//2
    left=InversePairsCore(data,datacopy,start,mid)
    right=InversePairsCore(data,datacopy,mid+1,end)
    count=0
    i=mid
    j=end
    loccopy=end
    while i>=start and j>mid:
        if data[i]>data[j]:
            count += j-mid
            datacopy[loccopy]=data[i]
            i -= 1
            loccopy -=1 
        else:
            datacopy[loccopy]=data[j]
            j -= 1
            loccopy -=1
    while i>=start:
        datacopy[loccopy]=data[i]
        i-=1
        loccopy-=1
    while j>mid:
        datacopy[loccopy]=data[j]
        j-=1
        loccopy-=1
    s=start
    while s<=end:
        data[s]=datacopy[s]
        s+=1
    return left + right + count
data=[7,5,6,4]
InversePairs(data)