K-complementary

gistfile1.java

import java.util.Arrays;

class Solution {
    public int solution(int K, int[] A) {
  	Arrays.sort(A);
        return getKC(K, A);
    }
    
    public int getKC(int K, int[] A) {

        int hit = 0;
        int left = 0;
        int right = A.length - 1;

        // scan from right and from left, to the opposite direction
		    // left -----> <------ right
        while(left <=  right) 
        {
        	int tk = A[left] + A[right];
            // K!
            if(tk == K) 
            {
                hit++;
                
                // vice versa. (P,Q) pair, so (Q,P) hits one more time.
                if(left!=right) hit++;  
                
                if(A[left]==A[left+1]) {
                    left++;
                }
                else if(A[right]==A[right-1]) 
                {
                    right++;
                }
                else 
                {
                	left++;
                    right--; 
                }              
            } else if(tk < K) { // less than K
                left++;                
            } else { // greater than K
                right--;                                
            }           
        }
        
        return hit;
        
    }
}