Even Frequency - Facebook Top Interview Questions

Problem Statement :

Given a list of integers nums, return whether all numbers appear an even number of times.

This should be done in \mathcal{O}(1)O(1) space.


n ≤ 100,000 where n is the length of nums

Example 1


nums = [2, 4, 4, 2, 3, 3]




Every number occurs twice.

Example 2


nums = [1]




1 occurs an odd number of times.

Solution :

                        Solution in C++ :

bool solve(vector<int>& nums) {
    int a = 0, b = 0;
    for (auto x : nums) {
        a ^= x;
        b ^= x + 1;
    return !a && !b;

                        Solution in Java :

import java.util.*;

class Solution {
    public boolean solve(int[] nums) {
        if (nums.length == 0 || nums.length % 2 == 1) {
            return false;

        for (int i = 0; i < nums.length; i += 2) {
            if (nums[i] != nums[i + 1]) {
                return false;
        return true;

                        Solution in Python : 
class Solution:
    def solve(self, nums):
        xor = 0
        for x in nums:
            # constants are primes arbitrarily taken from xxhash
            hash = ((x + 0x9E3779B1) * 0x85EBCA77) & 0xFFFFFFFF
            xor ^= hash
        return xor == 0

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