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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].

思路：用一个hash表存储每个元素及其相应的index，并在数组中进行循环查找

vector
   
     twoSum(vector
    
     & nums, int target) {    if (nums.size() <= 1)return vector
     
      {};    unordered_map
      
        res;    vector
       
         idx;    for (int i = 0; i < nums.size(); i++)res[nums[i]] = i;    for (int i = 0; i < nums.size(); i++){        int tmp = target - nums[i];        if (res.find(tmp) != res.end() && res[tmp] > i){            idx.push_back(i), idx.push_back(res[tmp]);            return idx;        }    }    return idx;}
       
      
     
    
   

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