Count the Number of Fair Pairs — LeetCode 2563 Python Solution

The problem

Given a 0-indexed integer array nums of size n and two integers lower and upper, return the number of fair pairs. A pair (i, j) is fair if: 0 <= i < j < n, and lower <= nums[i] + nums[j] <= upper

This statement is abridged. Read the full problem on LeetCode.

Example

Input

nums = [0,1,7,4,4,5], lower = 3, upper = 6

Output

6

Explanation

There are 6 fair pairs: (0,3), (0,4), (0,5), (1,3), (1,4), and (1,5).

Python solution

class Solution:
    def countFairPairs(self, nums: List[int], lower: int, upper: int) -> int:
        nums.sort()
        ans = 0
        for i, x in enumerate(nums):
            j = bisect_left(nums, lower - x, lo=i + 1)
            k = bisect_left(nums, upper - x + 1, lo=i + 1)
            ans += k - j
        return ans

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 2563. Count the Number of Fair Pairs is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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