For two sum, we can sort the array and scan from two ends. 

Then space O(1), time O(nlogn)

For here, it only accepts index. So we cant sort it as the index will change. Use hash table to record it.

Then space O(n), time O(n)

 

1 class Solution { 2 public: 3     vector
     
       twoSum(vector
      
        &numbers, int target) { 4         vector
       
         result(2); 5         unordered_map
        
          record; 6         for (int i = 0; i < numbers.size(); i++) { 7             if (record[target - numbers[i]]) { 8                 result[0] = record[target - numbers[i]]; 9                 result[1] = i+1;10                 return result;11             } else record[numbers[i]] = i+1;12         }13     }14 };