2341

2341. Maximum Number of Pairs in Array (Easy)

You are given a 0-indexed integer array nums. In one operation, you may do the following:

• Choose two integers in nums that are equal.
• Remove both integers from nums, forming a pair.

The operation is done on nums as many times as possible.

Return a 0-indexed integer array answer of size 2 where answer[0] is the number of pairs that are formed and answer[1] is the number of leftover integers in nums after doing the operation as many times as possible.

 

Example 1:

Input: nums = [1,3,2,1,3,2,2]
Output: [3,1]
Explanation:
Form a pair with nums[0] and nums[3] and remove them from nums. Now, nums = [3,2,3,2,2].
Form a pair with nums[0] and nums[2] and remove them from nums. Now, nums = [2,2,2].
Form a pair with nums[0] and nums[1] and remove them from nums. Now, nums = [2].
No more pairs can be formed. A total of 3 pairs have been formed, and there is 1 number leftover in nums.

Example 2:

Input: nums = [1,1]
Output: [1,0]
Explanation: Form a pair with nums[0] and nums[1] and remove them from nums. Now, nums = [].
No more pairs can be formed. A total of 1 pair has been formed, and there are 0 numbers leftover in nums.

Example 3:

Input: nums = [0]
Output: [0,1]
Explanation: No pairs can be formed, and there is 1 number leftover in nums.

 

Constraints:

• 1 <= nums.length <= 100
• 0 <= nums[i] <= 100

Companies: Amazon, Altimetrik

Related Topics:
Array, Hash Table, Counting

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Solution 1.

// OJ: https://leetcode.com/problems/maximum-number-of-pairs-in-array
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(D) where D is the range of the numbers
class Solution {
public:
    vector<int> numberOfPairs(vector<int>& A) {
        unordered_map<int, int> cnt;
        for (int n : A) cnt[n]++;
        int pair = 0, single = 0;
        for (auto &[n, c] : cnt) {
            pair += c / 2;
            single += c % 2;
        }
        return {pair, single};
    }
};