1679. Max Number of K-Sum Pairs

题目：

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 <= 10^5
• 1 <= nums[i] <= 10^9
• 1 <= k <= 10^9

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思路：

比较新的contest题，看上去应该是第二题，求给定数组中有几对数可以组成给定值k。用哈希表记录出现的次数，可以one pass。哈希表<int, int>记录出现的数字和次数。遍历数组，对于当前数字i，如果哈希表中存在k-i，则这是一对，更新答案并且删除哈希表中的这个数(如果出现次数大于1则减1，如果等于1则直接删除pair)，最后返回答案即可。

代码：

class Solution {
public:
    int maxOperations(vector<int>& nums, int k) {
        unordered_map<int,int> m;
        int count=0;
        for(auto i:nums)
        {
            if(m.count(k-i))
            {
                if(m[k-i]==1)
                    m.erase(k-i);
                else
                    m[k-i]--;
                count++;
            }
            else
                m[i]++;
        }
        return count;
    }
};