Given an array of integers, return indices of the two numbers such that they add up to a specific target.

You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example:

Given nums = [2, 7, 11, 15], target = 9,Because nums[0] + nums[1] = 2 + 7 = 9,return [0, 1]. Solution 1: use unordered_map to store 
    
     , find whether target-num exists in the map(O(1))
    

1 class Solution { 2 public: 3     vector
     
       twoSum(vector
      
       & nums, int target) { 4         unordered_map
       
         m; 5         vector
        
          res; 6         for (int i=0;i