title: Find The Element That Appears Once
signature: findTheElementThatAppearsOnce(arr, n) {
test_code: import 'dart:convert';
  
  // SOLUTION CODE
  // ============================================
  PLACEHOLDER_CODE_BODY
  
  // TESTING CODE 
  // ============================================
  bool validateSolution(int actual, int expected){
      return actual.toString() == expected.toString();
  }
  
  String driver(arr, n, int expected){
      try
      {
          if (validateSolution(PLACEHOLDER_FN_NAME(arr, n), expected)){
              return "PASSED";
          }
          return "FAILED";
      }catch(exception_object){
          return exception_object.runtimeType.toString();
      }
  }
  
  int main() {
  
      print("TEST-0...${driver(
          [7, 26, 26, 48, 59, 62, 66, 70, 72, 75, 76, 81, 97, 98], 7, 
          72)
      }");
  
      print("TEST-1...${driver(
          [-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)
      }");
  
      print("TEST-2...${driver(
          [0, 0, 1, 1], 3, 
          1)
      }");
  
      print("TEST-3...${driver(
          [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)
      }");
  
      print("TEST-4...${driver(
          [-98, -96, -92, -62, -52, -42, -42, -26, 4, 10, 14, 38, 64, 66, 72, 74, 82], 14, 
          104)
      }");
  
      print("TEST-5...${driver(
          [0, 1, 1, 1, 0, 0, 0, 1, 0, 1], 5, 
          0)
      }");
  
      print("TEST-6...${driver(
          [53, 63, 63], 2, 
          10)
      }");
  
      print("TEST-7...${driver(
          [-96, -38, -26, -46, 68, -36, 20, -18, -10, 52, 40, 94, -8, -64, 82, -22], 15, 
          34)
      }");
  
      print("TEST-8...${driver(
          [0, 0, 0, 0, 0, 1, 1], 3, 
          0)
      }");
  
      print("TEST-9...${driver(
          [99, 46, 48, 81, 27, 97, 26, 50, 77, 32, 45, 99, 46], 12, 
          1)
      }");
  
      return 0;
  }
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;
  }