find-k-th-smallest-pair-distance.py

# Time:  O(nlogn + nlogw), n = len(nums), w = max(nums)-min(nums)
# Space: O(1)

# Given an integer array, return the k-th smallest distance among all the pairs.
# The distance of a pair (A, B) is defined as the absolute difference between A and B.
#
# Example 1:
# Input:
# nums = [1,3,1]
# k = 1
# Output: 0 
# Explanation:
# Here are all the pairs:
# (1,3) -> 2
# (1,1) -> 0
# (3,1) -> 2
# Then the 1st smallest distance pair is (1,1), and its distance is 0.
#
# Note:
# 2 <= len(nums) <= 10000.
# 0 <= nums[i] < 1000000.
# 1 <= k <= len(nums) * (len(nums) - 1) / 2.

# Binary search with sliding window solution
class Solution(object):
    def smallestDistancePair(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: int
        """
        # Sliding window solution
        def possible(guess, nums, k):
            count, left = 0, 0
            for right, num in enumerate(nums):
                while num-nums[left] > guess:
                    left += 1
                count += right-left
            return count >= k

        nums.sort()
        left, right = 0, nums[-1]-nums[0]+1
        while left < right:
            mid = left + (right-left)/2
            if possible(mid, nums, k):
                right = mid
            else:
                left = mid+1
        return left