title: Find The Element That Appears Once
signature: function findTheElementThatAppearsOnce($arr, $n) {
test_code: <?php
  
  
  // SOLUTION CODE 
  // ============================================
  PLACEHOLDER_CODE_BODY
  
  // TESTING CODE 
  // ============================================
  function validateSolution($actual, $expected){
      return serialize($actual) == serialize($expected);
  }
  
  function driver($arr, $n, $expected){
      try{
        if (validateSolution(PLACEHOLDER_FN_NAME($arr, $n), $expected)){
          return "PASSED";
        }
        return "FAILED";
      }
      catch (Exception $exception_obj) {
        return get_class($exception_obj);
      }
  }
  
  function main(){
      printf("TEST-0...%s\n",driver(
        array(7, 26, 26, 48, 59, 62, 66, 70, 72, 75, 76, 81, 97, 98), 7, 
        72));
      
      printf("TEST-1...%s\n",driver(
        array(-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));
      
      printf("TEST-2...%s\n",driver(
        array(0, 0, 1, 1), 3, 
        1));
      
      printf("TEST-3...%s\n",driver(
        array(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));
      
      printf("TEST-4...%s\n",driver(
        array(-98, -96, -92, -62, -52, -42, -42, -26, 4, 10, 14, 38, 64, 66, 72, 74, 82), 14, 
        104));
      
      printf("TEST-5...%s\n",driver(
        array(0, 1, 1, 1, 0, 0, 0, 1, 0, 1), 5, 
        0));
      
      printf("TEST-6...%s\n",driver(
        array(53, 63, 63), 2, 
        10));
      
      printf("TEST-7...%s\n",driver(
        array(-96, -38, -26, -46, 68, -36, 20, -18, -10, 52, 40, 94, -8, -64, 82, -22), 15, 
        34));
      
      printf("TEST-8...%s\n",driver(
        array(0, 0, 0, 0, 0, 1, 1), 3, 
        0));
      
      printf("TEST-9...%s\n",driver(
        array(99, 46, 48, 81, 27, 97, 26, 50, 77, 32, 45, 99, 46), 12, 
        1));
      
  }
  
  main();
  ?>
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;
  }