Sum of array elements that is first continuously increasing then decreasing | Efficient Javascript method to find sum of the elements of array that is halfway increasing and then halfway decreassing ; Driver code
function arraySum ( arr , n ) { let x = ( n + 1 ) / 2 ; return ( arr [ 0 ] - 1 ) * n + x * x ; } let arr = [ 10 , 11 , 12 , 13 , 12 , 11 , 10 ] ; let n = arr . length ; document . write ( arraySum ( arr , n ) ) ;