title: Find The Element That Appears Once
signature: def findTheElementThatAppearsOnce(arr: List[Int], n: Int) = {
test_code: import scala.collection.mutable.HashMap
  import scala.collection.immutable.HashSet
  
  
  // SOLUTION CODE
  // ============================================
  PLACEHOLDER_CODE_BODY
  
  
  // TESTING CODE 
  // ============================================
  def validateSolution(actual: Int , expected: Int): Boolean = {
      actual.toString == expected.toString
  }
  
  def driver(arr: List[Int], n: Int, expected: Int): String = {
      try {
          if (validateSolution(PLACEHOLDER_FN_NAME(arr, n),expected)) {
              "PASSED"
          } else{
              "FAILED"
          }
      }catch{
          case e: Exception => {e.getClass.getName}
      }
  
  }
  object QuestionEvaluator extends App {
  
      println(s"TEST-0...${driver(
        List(7, 26, 26, 48, 59, 62, 66, 70, 72, 75, 76, 81, 97, 98), 7, 
        72    
      )}")   
  
  
      println(s"TEST-1...${driver(
        List(-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(s"TEST-2...${driver(
        List(0, 0, 1, 1), 3, 
        1    
      )}")   
  
  
      println(s"TEST-3...${driver(
        List(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(s"TEST-4...${driver(
        List(-98, -96, -92, -62, -52, -42, -42, -26, 4, 10, 14, 38, 64, 66, 72, 74, 82), 14, 
        104    
      )}")   
  
  
      println(s"TEST-5...${driver(
        List(0, 1, 1, 1, 0, 0, 0, 1, 0, 1), 5, 
        0    
      )}")   
  
  
      println(s"TEST-6...${driver(
        List(53, 63, 63), 2, 
        10    
      )}")   
  
  
      println(s"TEST-7...${driver(
        List(-96, -38, -26, -46, 68, -36, 20, -18, -10, 52, 40, 94, -8, -64, 82, -22), 15, 
        34    
      )}")   
  
  
      println(s"TEST-8...${driver(
        List(0, 0, 0, 0, 0, 1, 1), 3, 
        0    
      )}")   
  
  
      println(s"TEST-9...${driver(
        List(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;
  }