2206. Divide Array Into Equal Pairs

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Array Hash Table Sorting Bit Manipulation Counting

Time: O(n)

Space: O(n)

Intuition

Count pairs with equal elements. For each value, count occurrences - pairs = count*(count-1)/2.

Algorithm

1Frequency map. For each value v with count c: pairs += c*(c-1)/2.

Common Pitfalls

•C(n,2) = n*(n-1)/2 gives number of pairs from n elements.

2206.cs

// Approach: Count occurrences; all counts must be even to form pairs.
// Time: O(n) Space: O(n)

public class Solution
{
    public bool DivideArray(int[] nums)
    {
        int[] count = new int[501];

        foreach (var num in nums)
            ++count[num];

        return count.All(c => c % 2 == 0);
    }
}

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