Count pairs in a sorted array whose sum is less than x

//first approach is brute force
class Solution:
    #Complete the below function
    def countPairs(self, arr, target):
        arr=sorted(arr)
        ans=0
        i=0
        j=len(arr)-1
        while i<j:
            if arr[i]+arr[j]>=target:
                j-=1
            else:
                ans+= (j-i)
                i+=1
        return ans

// If current left and current right have sum smaller than x,
// the all elements from l+1 to r form a pair with current l.