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.

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> hash;
        vector<int> newsum;
        int i,j;
        for(i=0;i<nums.size();i++)
        {
            if(hash.find(target-nums[i])!=hash.end())
            {
                newsum.push_back(hash[target-nums[i]]);
                newsum.push_back(i);
            }
            else
            {
                hash[nums[i]]=i;
            }
        }

        return newsum;
    }
};

思路：就是遍历一遍所有数据，用target数据减去遍历的数据，然后假如减去后得到的数据是在哈希表里的，那么就说明得到了我们想要的东西。假如没有在哈希表中找到，那么就把这个数放入哈希表里。