2206. Divide Array Into Equal Pairs

Method 1

You are given an integer array nums consisting of 2 * n integers.
You need to divide nums into n pairs such that:
- Each element belongs to exactly one pair.
- The elements present in a pair are equal.
Return true if nums can be divided into n pairs, otherwise return false.

Example 1

Input: nums = [3,2,3,2,2,2]
Output: true
Explanation: 
There are 6 elements in nums, so they should be divided into 6 / 2 = 3 pairs.
If nums is divided into the pairs (2, 2), (3, 3), and (2, 2), it will satisfy all the conditions.

Example 2

Input: nums = [1,2,3,4]
Output: false
Explanation: 
There is no way to divide nums into 4 / 2 = 2 pairs such that the pairs satisfy every condition.

Method 1

【O(n) time | O(1) space】

package Leetcode.Math;

/**
 * @author zhengxingxing
 * @date 2025/03/17
 */
public class DivideArrayIntoEqualPairs {
    
    public static boolean divideArray(int[] nums) {
        // Create counting array (constraint: nums[i] <= 500)
        int[] count = new int[501];

        // Count frequency of each number
        for (int num : nums) {
            count[num]++;
        }

        // Check if each number appears even number of times
        for (int c : count) {
            if (c % 2 != 0) {
                return false;  // Found a number with odd frequency
            }
        }

        return true;  // All numbers appear even number of times
    }
    
    public static void main(String[] args) {
        // Test Case 1: Should return true
        int[] nums1 = {3,2,3,2,2,2};
        System.out.println("Test Case 1: ");
        System.out.print("Input array: ");
        printArray(nums1);
        System.out.println("Expected result: true");
        System.out.println("Actual result: " + divideArray(nums1));

        // Test Case 2: Should return false
        int[] nums2 = {1,2,3,4};
        System.out.println("\nTest Case 2: ");
        System.out.print("Input array: ");
        printArray(nums2);
        System.out.println("Expected result: false");
        System.out.println("Actual result: " + divideArray(nums2));
    }
    
    private static void printArray(int[] arr) {
        System.out.print("[");
        for (int i = 0; i < arr.length; i++) {
            System.out.print(arr[i]);
            if (i < arr.length - 1) {
                System.out.print(", ");
            }
        }
        System.out.println("]");
    }
}

Method 1

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