【朱迪的LeetCode刷题笔记】】1679. Max Number of K-Sum Pairs #Easy #Python

Judie

You are given an integer array nums and an integer k.

In one operation, you can pick two numbers from the array whose sum equals k and remove them from the array.

Return the maximum number of operations you can perform on the array.

Example 1:
Input: nums = [1,2,3,4], k = 5
Output: 2
Explanation: Starting with nums = [1,2,3,4]:
- Remove numbers 1 and 4, then nums = [2,3]
- Remove numbers 2 and 3, then nums = []
There are no more pairs that sum up to 5, hence a total of 2 operations.

Example 2:
Input: nums = [3,1,3,4,3], k = 6
Output: 1
Explanation: Starting with nums = [3,1,3,4,3]:
- Remove the first two 3's, then nums = [1,4,3]
There are no more pairs that sum up to 6, hence a total of 1 operation.

Constraints:
1 <= nums.length <= 105
1 <= nums <= 109
1 <= k <= 109

Judy
Python hash table

1. class Solution(object):
   
2.     def maxOperations(self, nums, k):
   
3.         """
   
4.         :type nums: List[int]
   
5.         :type k: int
   
6.         :rtype: int
   
7.         """
   
8.         d = {}
   
9.         c = 0
   
10.         for i, x in enumerate(nums):
    
11.             if x in d and d[x] > 0:
    
12.                 c += 1
    
13.                 d[x] -= 1
    
14.             elif k-x in d:
    
15.                 d[k-x] += 1
    
16.             else:
    
17.                 d[k-x] = 1
    
18.         return c
    

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Sol1
Python slower? but oneliner! lol
https://leetcode.com/problems/ma ... p;envId=leetcode-75

1. class Solution:
   
2.     def maxOperations(self, nums, k):
   
3.         cnt, ans = Counter(nums), 0
   
4.         for val in cnt:
   
5.             ans += min(cnt[val], cnt[k - val])
   
6.         return ans//2

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1. return (lambda c: sum(min(c[n], c[k-n]) for n in c))(Counter(nums))//2

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