# 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.

Constraints

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

Example 1

Input

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

Output

True

Explanation

Every number occurs twice.

Example 2

Input

nums = [1]

Output

False

Explanation

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;
}

Arrays.sort(nums);
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


## Sparse Arrays

There is a collection of input strings and a collection of query strings. For each query string, determine how many times it occurs in the list of input strings. Return an array of the results. Example: strings=['ab', 'ab', 'abc'] queries=['ab', 'abc', 'bc'] There are instances of 'ab', 1 of 'abc' and 0 of 'bc'. For each query, add an element to the return array, results=[2,1,0]. Fun

## Array Manipulation

Starting with a 1-indexed array of zeros and a list of operations, for each operation add a value to each of the array element between two given indices, inclusive. Once all operations have been performed, return the maximum value in the array. Example: n=10 queries=[[1,5,3], [4,8,7], [6,9,1]] Queries are interpreted as follows: a b k 1 5 3 4 8 7 6 9 1 Add the valu

## Print the Elements of a Linked List

This is an to practice traversing a linked list. Given a pointer to the head node of a linked list, print each node's data element, one per line. If the head pointer is null (indicating the list is empty), there is nothing to print. Function Description: Complete the printLinkedList function in the editor below. printLinkedList has the following parameter(s): 1.SinglyLinkedListNode

## Insert a Node at the Tail of a Linked List

You are given the pointer to the head node of a linked list and an integer to add to the list. Create a new node with the given integer. Insert this node at the tail of the linked list and return the head node of the linked list formed after inserting this new node. The given head pointer may be null, meaning that the initial list is empty. Input Format: You have to complete the SinglyLink