title: Find The Element That Appears Once
signature: pub fn find_the_element_that_appears_once(arr: Vec<i32>, n: i32) -> i32 {
test_code: use std::collections::HashMap;
  use std::collections::HashSet;
  use std::cmp::*;
  use std::collections::*;
  use std::fmt::*; 
  
  
  // SOLUTION CODE
  // ============================================
  PLACEHOLDER_CODE_BODY
  
  // TESTING CODE 
  // ============================================
  pub fn validate_solution(actual: i32, expected: i32)->bool{
  return actual==expected
  }
  pub fn driver(arr: Vec<i32>, n: i32, expected: i32)->String{
      if validate_solution(PLACEHOLDER_FN_NAME(arr, n), expected) {
        return "PASSED".to_string();
      };
      return "FAILED".to_string();
  
  }
  
  fn main() {
      println!("TEST-0...{}", driver(
        Vec::from([7, 26, 26, 48, 59, 62, 66, 70, 72, 75, 76, 81, 97, 98]), 7, 
        72
      ));
      
      println!("TEST-1...{}", driver(
        Vec::from([-42, -48, -64, -74, 56, -34, 20, 16, 34, -84, 86, 38, 56, -86, 30, -74, -96, 96, 12, 10, -46, 10, -36, 38, 34, -46, -20, 14, 12, 62, -54, 20, -82, 24, 96]), 27, 
        -28
      ));
      
      println!("TEST-2...{}", driver(
        Vec::from([0, 0, 1, 1]), 3, 
        1
      ));
      
      println!("TEST-3...{}", driver(
        Vec::from([68, 91, 61, 6, 32, 47, 76, 69, 44, 71, 29, 79, 74, 33, 44, 33, 45, 75, 43, 82, 83, 81, 95, 16, 86, 33, 69, 61, 73, 21, 54, 17, 98, 62, 14, 72, 80, 31, 56, 82, 14, 48, 76]), 38, 
        25
      ));
      
      println!("TEST-4...{}", driver(
        Vec::from([-98, -96, -92, -62, -52, -42, -42, -26, 4, 10, 14, 38, 64, 66, 72, 74, 82]), 14, 
        104
      ));
      
      println!("TEST-5...{}", driver(
        Vec::from([0, 1, 1, 1, 0, 0, 0, 1, 0, 1]), 5, 
        0
      ));
      
      println!("TEST-6...{}", driver(
        Vec::from([53, 63, 63]), 2, 
        10
      ));
      
      println!("TEST-7...{}", driver(
        Vec::from([-96, -38, -26, -46, 68, -36, 20, -18, -10, 52, 40, 94, -8, -64, 82, -22]), 15, 
        34
      ));
      
      println!("TEST-8...{}", driver(
        Vec::from([0, 0, 0, 0, 0, 1, 1]), 3, 
        0
      ));
      
      println!("TEST-9...{}", driver(
        Vec::from([99, 46, 48, 81, 27, 97, 26, 50, 77, 32, 45, 99, 46]), 12, 
        1
      ));
      
  }
source_py: def find_the_element_that_appears_once(arr, n):
      ones = 0
      twos = 0
      for i in range(n):
          twos = (twos | (ones & arr[i]))
          ones = (ones ^ arr[i])
          common_bit_mask = (~ (ones & twos))
          ones &= common_bit_mask
          twos &= common_bit_mask
      return ones
source_cpp: int find_the_element_that_appears_once(vector<int> arr, int n) {
    int ones = 0, twos = 0;
    int common_bit_mask;
    for ( int i = 0;
    i < n;
    i ++ ) {
      twos = twos | ( ones & arr [ i ] );
      ones = ones ^ arr [ i ];
      common_bit_mask = ~ ( ones & twos );
      ones &= common_bit_mask;
      twos &= common_bit_mask;
    }
    return ones;
  }