title: Sum Fai Aj Pairs Array N Integers
signature: def sumFaiAjPairsArrayNIntegers(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(2, 8, 12, 19, 23, 23, 26, 39, 54, 56, 57, 57, 73, 78, 83, 83, 89, 91), 15, 
        3278    
      )}")   
  
  
      println(s"TEST-1...${driver(
        List(62, -34, 10, -28, -42, -12, 4, 20, -20, -84, -76, -16, -44, 26, -78, -40, 50, -10, -56, 76, -88, 24, 64, 10, 64, -8, -68, -42, 26, 24, 62, 36, -68, 8, -68, -2, 8, 38, -18), 20, 
        -112    
      )}")   
  
  
      println(s"TEST-2...${driver(
        List(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), 18, 
        0    
      )}")   
  
  
      println(s"TEST-3...${driver(
        List(23, 14, 44, 29, 93, 56, 22, 29, 97, 71, 43, 72, 74, 8, 92, 40, 18, 34, 78, 79, 93, 63, 79, 44, 35, 72, 88, 83, 40, 89, 66, 66, 54, 56, 44, 62, 72, 94, 79, 79, 24, 55, 72, 37, 27, 55, 16, 58, 83), 25, 
        2551    
      )}")   
  
  
      println(s"TEST-4...${driver(
        List(-96, -94, -90, -90, -86, -82, -80, -80, -58, -48, -40, -32, -32, -20, -20, -20, -12, -12, -6, -6, -2, 0, 4, 16, 16, 16, 42, 48, 58, 64, 68, 76, 76, 76, 78, 80, 82, 88, 88, 88, 92, 94, 96, 98), 27, 
        16640    
      )}")   
  
  
      println(s"TEST-5...${driver(
        List(1, 1, 1, 1, 0, 0, 0, 1, 1, 0, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 1, 1, 1, 0, 1, 0, 0, 0, 1, 1, 1, 0, 0, 1, 1, 1, 1), 34, 
        0    
      )}")   
  
  
      println(s"TEST-6...${driver(
        List(3, 4, 4, 8, 10, 10, 11, 15, 19, 19, 20, 24, 25, 25, 27, 30, 41, 43, 44, 45, 47, 55, 59, 59, 61, 63, 63, 67, 67, 69, 72, 73, 75, 77, 77, 78, 81, 81, 83, 84, 92, 94, 99), 31, 
        12408    
      )}")   
  
  
      println(s"TEST-7...${driver(
        List(94, -86, 94, 54, -52, 86, 68, 64, 98, 54, -14, -60, -60, -92, 80, -16, 28, 16, -74, 68, 32, -54, 58, -16, -2, -52, -92, -36, 96, -18, 14, 76, 16), 26, 
        -6856    
      )}")   
  
  
      println(s"TEST-8...${driver(
        List(0, 0, 0, 0, 1, 1, 1, 1), 7, 
        0    
      )}")   
  
  
      println(s"TEST-9...${driver(
        List(10, 28, 63, 2, 78, 12, 51, 82, 89, 65, 99), 8, 
        542    
      )}")   
  
  }
  
source_py: def sum_fai_aj_pairs_array_n_integers(a, n):
      cnt = dict()
      ans = 0
      pre_sum = 0
      for i in range(n):
          ans += ((i * a[i]) - pre_sum)
          pre_sum += a[i]
          if ((a[i] - 1) in cnt):
              ans -= cnt[(a[i] - 1)]
          if ((a[i] + 1) in cnt):
              ans += cnt[(a[i] + 1)]
          if (a[i] not in cnt):
              cnt[a[i]] = 0
          cnt[a[i]] += 1
      return ans
source_cpp: int sum_fai_aj_pairs_array_n_integers(vector<int> a, int n) {
    unordered_map < int, int > cnt;
    int ans = 0, pre_sum = 0;
    for ( int i = 0;
    i < n;
    i ++ ) {
      ans += ( i * a [ i ] ) - pre_sum;
      pre_sum += a [ i ];
      if ( cnt [ a [ i ] - 1 ] ) ans -= cnt [ a [ i ] - 1 ];
      if ( cnt [ a [ i ] + 1 ] ) ans += cnt [ a [ i ] + 1 ];
      cnt [ a [ i ] ] ++;
    }
    return ans;
  }