Count Triplets such that one of the numbers can be written as sum of the other two | Function to count the number of ways to choose the triples ; compute the max value in the array and create frequency array of size max_val + 1. We can also use HashMap to store frequencies . We have used an array to keep remaining code simple . ; Case 1 : 0 , 0 , 0 ; Case 2 : 0 , x , x ; Case 3 : x , x , 2 * x ; Case 4 : x , y , x + y iterate through all pairs ( x , y ) ; Driver code
function countWays ( arr , n ) { let max_val = 0 ; for ( let i = 0 ; i < n ; i ++ ) max_val = Math . max ( max_val , arr [ i ] ) ; let freq = new Array ( max_val + 1 ) ; for ( let i = 0 ; i < freq . length ; i ++ ) { freq [ i ] = 0 ; } for ( let i = 0 ; i < n ; i ++ ) freq [ arr [ i ] ] ++ ; ans += freq [ 0 ] * ( freq [ 0 ] - 1 ) * ( freq [ 0 ] - 2 ) / 6 ; for ( let i = 1 ; i <= max_val ; i ++ ) ans += freq [ 0 ] * freq [ i ] * ( freq [ i ] - 1 ) / 2 ; for ( let i = 1 ; 2 * i <= max_val ; i ++ ) ans += freq [ i ] * ( freq [ i ] - 1 ) / 2 * freq [ 2 * i ] ; for ( let i = 1 ; i <= max_val ; i ++ ) { for ( let j = i + 1 ; i + j <= max_val ; j ++ ) ans += freq [ i ] * freq [ j ] * freq [ i + j ] ; } return ans ; } let arr = [ 1 , 2 , 3 , 4 , 5 ] ; let n = arr . length ; document . write ( countWays ( arr , n ) ) ;