1. Two Sum

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].

 

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        unordered_map<int,int>mapping;
        vector<int> res;
        for(int i=0;i<nums.size();i++)
            mapping[nums[i]]=i;
        for(int i=0;i<nums.size();i++)
        {
            int gap =target-nums[i];
            if(mapping.find(gap)!=mapping.end()&&mapping[gap]>i)
                //后面半句是为了防止一个数用两次
            {
               res.push_back(i); 
                res.push_back(mapping[gap]);
                break;
            }      
        }
        return res;
        
    }
};