leetcode - 373. Find K Pairs with Smallest Sums

Description

You are given two integer arrays nums1 and nums2 sorted in ascending order and an integer k.

Define a pair (u, v) which consists of one element from the first array and one element from the second array.

Return the k pairs (u1, v1), (u2, v2), …, (uk, vk) with the smallest sums.

Example 1:

Input: nums1 = [1,7,11], nums2 = [2,4,6], k = 3
Output: [[1,2],[1,4],[1,6]]
Explanation: The first 3 pairs are returned from the sequence: [1,2],[1,4],[1,6],[7,2],[7,4],[11,2],[7,6],[11,4],[11,6]

Example 2:

Input: nums1 = [1,1,2], nums2 = [1,2,3], k = 2
Output: [[1,1],[1,1]]
Explanation: The first 2 pairs are returned from the sequence: [1,1],[1,1],[1,2],[2,1],[1,2],[2,2],[1,3],[1,3],[2,3]

Example 3:

Input: nums1 = [1,2], nums2 = [3], k = 3
Output: [[1,3],[2,3]]
Explanation: All possible pairs are returned from the sequence: [1,3],[2,3]

Constraints:

1 <= nums1.length, nums2.length <= 10^5
-10^9 <= nums1[i], nums2[i] <= 10^9
nums1 and nums2 both are sorted in ascending order.
1 <= k <= 10^4

Solution

When the current pair is (i, j), the next optimal pair could appear either (i, j+1) or (i+1, j). Use a heap to keep track of all the possible candidates.

Time complexity: $o(n\log n)$
Space complexity: $o(n)$

Code

class Solution:
    def kSmallestPairs(self, nums1: List[int], nums2: List[int], k: int) -> List[List[int]]:
        import heapq
        heap = [(nums1[0] + nums2[0], 0, 0)]
        res = []
        visited = set()
        while heap and len(res) < k:
            _, i, j = heapq.heappop(heap)
            if (i, j) in visited:
                continue
            res.append([nums1[i], nums2[j]])
            visited.add((i, j))
            if j + 1 < len(nums2) and (i, j + 1) not in visited:
                heapq.heappush(heap, (nums1[i] + nums2[j + 1], i, j + 1))
            if i + 1 < len(nums1) and (i + 1, j) not in visited:
                heapq.heappush(heap, (nums1[i + 1] + nums2[j], i + 1, j))
        return res