Find number of subarrays with even sum | Java program to count number of sub - arrays with even sum using an efficient algorithm Time Complexity - O ( N ) Space Complexity - O ( 1 ) ; result may be large enough not to fit in int ; ; to keep track of subarrays with even sum starting from index i ; if a [ i ] is odd then all subarrays starting from index i + 1 which was odd becomeseven when a [ i ] gets added to it . ; if a [ i ] is even then all subarrays starting from index i + 1 which was even remainseven and one extra a [ i ] even subarray gets added to it . ; Driver Code
import java . io . * ; class GFG { public static long countEvenSum ( int a [ ] , int n ) { long res = 0 ; int s = 0 ; for ( int i = n - 1 ; i >= 0 ; i -- ) { if ( a [ i ] % 2 == 1 ) { s = n - i - 1 - s ; } else { s = s + 1 ; } res = res + s ; } return res ; } public static void main ( String [ ] args ) { int arr [ ] = { 1 , 2 , 2 , 3 , 4 , 1 } ; int n = arr . length ; System . out . println ( " The ▁ Number ▁ of ▁ Subarrays " + " ▁ with ▁ even ▁ sum ▁ is ▁ " + countEvenSum ( arr , n ) ) ; } }