输入: A = [2, 4, 1, 3, 5]
输出: 3
解释:
(2, 1), (4, 1), (4, 3) 是逆序对
class Solution:
# @param {int[]} A an array
# @return {int} total of reverse pairs
def reversePairs(self, A):
# Write your code here
self.tmp = [0] * len(A)
return self.mergeSort(A, 0, len(A) - 1)
def mergeSort(self, A, l, r):
if l >= r:
return 0
m = (l + r) >> 1
ans = self.mergeSort(A, l, m) + self.mergeSort(A, m + 1, r)
i, j, k = l, m + 1, l
while i <= m and j <= r:
if A[i] > A[j]:
self.tmp[k] = A[j]
j += 1
ans += m - i + 1
else:
self.tmp[k] = A[i]
i += 1
k += 1
while i <= m:
self.tmp[k] = A[i]
k += 1
i += 1
while j <= r:
self.tmp[k] = A[j]
k += 1
j += 1
for i in xrange(l, r + 1):
A[i] = self.tmp[i]
return ans
