class Solution:
    def find_reversed_pairs(self,nums,left,right):
        res,mid = 0,(left+right)//2
        
        j = mid+1
        for i in range(left,mid+1):
            while j <= right and nums[i] > 2*nums[j]:
                res += mid-i+1
                j += 1
        return res      
        
    def merge_sort(self,nums,nums_sorted,l,r):
        if l >= r: return 0
        mid = (l+r) // 2
        res = self.merge_sort(nums,nums_sorted,l,mid) +\
              self.merge_sort(nums,nums_sorted,mid+1,r) +\
              self.find_reversed_pairs(nums,l,r)
        
        i,j,k = l,mid+1,l
        while i <= mid and j <= r:
            if nums[i] <= nums[j]:
                nums_sorted[k] = nums[i]
                i += 1
            else:
                nums_sorted[k] = nums[j]
                j += 1
            k += 1
                
        while i <= mid:
            nums_sorted[k] = nums[i]
            i += 1
            k += 1
        while j <= r:
            nums_sorted[k] = nums[j]
            j += 1
            k += 1
        
        for k in range(l,r+1): nums[k] = nums_sorted[k]
        
        return res
        
    
    def reversePairs(self, nums: List[int]) -> int:
        if not nums: return 0
        nums_sorted = [0] * len(nums)
        return self.merge_sort(nums,nums_sorted,0,len(nums)-1)