title: Find The Missing Number 1
signature: def findTheMissingNumber1(a: 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(a: List[Int], n: Int, expected: Int): String = {
      try {
          if (validateSolution(PLACEHOLDER_FN_NAME(a, n),expected)) {
              "PASSED"
          } else{
              "FAILED"
          }
      }catch{
          case e: Exception => {e.getClass.getName}
      }
  
  }
  object QuestionEvaluator extends App {
  
      println(s"TEST-0...${driver(
        List(13, 27, 46, 59, 62, 82, 92), 6, 
        -261    
      )}")   
  
  
      println(s"TEST-1...${driver(
        List(22, 86, -64, -20, -56, -16, 86, 42, 72, -90, 10, 42, 56, 8, 50, 24, -34, 0, -78, 64, 18, 20, -84, -22, 90, -20, 86, 26, -54, 0, 90, -48, 4, 88, 18, -64, -22, -74, 48, -36, -86, -24, 88, -64, 68, 62, 92), 38, 
        524    
      )}")   
  
  
      println(s"TEST-2...${driver(
        List(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1), 15, 
        131    
      )}")   
  
  
      println(s"TEST-3...${driver(
        List(55, 89, 56, 85, 26, 4, 91, 91, 3, 77, 63, 59, 76, 90, 1, 94, 44, 70, 8, 54, 3, 91, 29, 95, 28, 75, 20), 22, 
        -954    
      )}")   
  
  
      println(s"TEST-4...${driver(
        List(-94, -84, -80, -78, -66, -62, -54, -52, -26, -8, -8, -6, 4, 4, 8, 14, 26, 58, 60, 62, 62, 76, 78, 86, 92), 18, 
        694    
      )}")   
  
  
      println(s"TEST-5...${driver(
        List(1, 0, 0, 0, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0, 1, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0), 25, 
        341    
      )}")   
  
  
      println(s"TEST-6...${driver(
        List(1, 2, 7, 7, 9, 14, 23, 29, 31, 31, 35, 35, 38, 41, 44, 49, 49, 50, 51, 54, 55, 56, 57, 63, 67, 69, 73, 79, 79, 80, 86, 88, 93), 24, 
        -506    
      )}")   
  
  
      println(s"TEST-7...${driver(
        List(78, -48, 16, 22, -16, 34, 56, -20, -62, -82, -74, -40, 20, -24, -46, 64, 66, -76, 58, -84, 96, 76, 86, -32, 46), 12, 
        227    
      )}")   
  
  
      println(s"TEST-8...${driver(
        List(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1), 29, 
        463    
      )}")   
  
  
      println(s"TEST-9...${driver(
        List(73, 76, 25, 59, 40, 85, 90, 38, 13, 97, 93, 99, 45, 7), 12, 
        -697    
      )}")   
  
  }
  
source_py: def find_the_missing_number_1(a, n):
      (i, total) = (0, 1)
      for i in range(2, (n + 2)):
          total += i
          total -= a[(i - 2)]
      return total
source_cpp: int find_the_missing_number_1(vector<int> a, int n) {
    int i, total = 1;
    for ( i = 2;
    i <= ( n + 1 );
    i ++ ) {
      total += i;
      total -= a [ i - 2 ];
    }
    return total;
  }