leetcode/hash-table/TwoSum.java

There is a better solution for the TwoSum problem than the hashmap solution. Sure hashmap is fine and meets the desired output. however it can be completely eliminated as such. The techinque employs a pointers to track the argument array i.e. one from the begining aka head and the other one from the end aka tail. A single sweep on the array and adjusting the two pointers does the job rather nicely.

       //input array must be sorted 
       int head =0;  int tail = arr.length -1;  int k = 11;  //target sum to find

        while(head < tail) {
           int sum = arr[head] + arr[tail];
           if(sum == k)  return true; //found it !!
           else if(sum < k) ++head;
           else --tail; 
        }