Count Pairs Whose Sum is Less than Target

//first approach is brute force
class Solution:
    def countPairs(self, nums: List[int], target: int) -> int:
        arr=sorted(nums)
        i,j=0,len(arr)-1
        ans=0
        while i<=j:
            if arr[i]+arr[j]>=target:
                j-=1
            elif arr[i]+arr[j]<target: #arr[i] can form valid pairs with every element from i+1 to j
                ans+= (j-i)
                i+=1
        return ans