Count of pairs in an array whose sum is a perfect square | Java implementation of the approach ; Function to return an ArrayList containing all the perfect squares upto n ; while current perfect square is less than or equal to n ; Function to print the sum of maximum two elements from the array ; Function to return the count of numbers that when added with n give a perfect square ; temp > n is checked so that pairs ( x , y ) and ( y , x ) don 't get counted twice ; Function to count the pairs whose sum is a perfect square ; Sum of the maximum two elements from the array ; List of perfect squares upto max ; Contains all the array elements ; Add count of the elements that when added with arr [ i ] give a perfect square ; Driver code
import java . util . * ; public class GFG { public static ArrayList < Integer > getPerfectSquares ( int n ) { ArrayList < Integer > perfectSquares = new ArrayList < > ( ) ; int current = 1 , i = 1 ; while ( current <= n ) { perfectSquares . add ( current ) ; current = ( int ) Math . pow ( ++ i , 2 ) ; } return perfectSquares ; } public static int maxPairSum ( int arr [ ] ) { int n = arr . length ; int max , secondMax ; if ( arr [ 0 ] > arr [ 1 ] ) { max = arr [ 0 ] ; secondMax = arr [ 1 ] ; } else { max = arr [ 1 ] ; secondMax = arr [ 0 ] ; } for ( int i = 2 ; i < n ; i ++ ) { if ( arr [ i ] > max ) { secondMax = max ; max = arr [ i ] ; } else if ( arr [ i ] > secondMax ) { secondMax = arr [ i ] ; } } return ( max + secondMax ) ; } public static int countPairsWith ( int n , ArrayList < Integer > perfectSquares , HashSet < Integer > nums ) { int count = 0 ; for ( int i = 0 ; i < perfectSquares . size ( ) ; i ++ ) { int temp = perfectSquares . get ( i ) - n ; if ( temp > n && nums . contains ( temp ) ) count ++ ; } return count ; } public static int countPairs ( int arr [ ] ) { int i , n = arr . length ; int max = maxPairSum ( arr ) ; ArrayList < Integer > perfectSquares = getPerfectSquares ( max ) ; HashSet < Integer > nums = new HashSet < > ( ) ; for ( i = 0 ; i < n ; i ++ ) nums . add ( arr [ i ] ) ; int count = 0 ; for ( i = 0 ; i < n ; i ++ ) { count += countPairsWith ( arr [ i ] , perfectSquares , nums ) ; } return count ; } public static void main ( String [ ] args ) { int arr [ ] = { 2 , 3 , 6 , 9 , 10 , 20 } ; System . out . println ( countPairs ( arr ) ) ; } }