java - Recursion: Determine if there are two elements in an array A that sum to k -

i've seen question asked lot. how works: check sum of first , last. if sum more k last-- or if less k first++. carries on recursively till either first == last or sum k found. **note values in array sorted in ascending order.

i've tried using recursion, whenever run it, returns "false". i've tried arrays of sizes , test cases return false. e.g. array[1 2 3 4 5 6], size n = 6 , k = 7 returns false when should true. cannot seem find bug. can please give me indication making mistake? , if i'm not mistaken runs in o(n) time?

public static boolean sum( int[] a, int n, int k ) //where k sum needed , n size of array {      if (k <= a[0] || k >= a[n]*2)      {           return false;      }      int = 0;      int j = n-1;      return sum_recursion(a, n, k, i, j); }   private static boolean sum_recursion(int[] a, int n, int k, int i, int j)  {     if(i == j)     {          return false;     } else if((a[i] + a[j]) == k)            {         return true;     } else if ((a[i] + a[j]) > k)     {         return sum_recursion(a, n, k, i, j--);     }         return sum_recursion(a, n, k, i++, j); } 

two problems:

1. the j--, use j first, --. should j - 1 or --j. same story i++.

2. the n parameter seems useless. when use it, index out bounds.

fixed version correct result:

public static void main(string[] args) {     int[] = {1, 2, 3, 4, 5, 6};     system.out.println(sum(a, 6, 7)); // ==> true }  public static boolean sum(int[] a, int n, int k) //where k sum needed , n size of array {     if (k <= a[0] || k >= a[n - 1] * 2) {         return false;     }     int = 0;     int j = n - 1;     return sum_recursion(a, n, k, i, j); }   private static boolean sum_recursion(int[] a, int n, int k, int i, int j) {     if (i == j) {         return false;     } else if ((a[i] + a[j]) == k) {         return true;     } else if ((a[i] + a[j]) > k) {         return sum_recursion(a, n, k, i, j - 1);     }     return sum_recursion(a, n, k, + 1, j); }