1 Two Sum

链接：https://leetcode.com/problems/two-sum/
问题描述：
Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

Hide Tags Array Hash Table

给出一个目标数，找到两个数，两数之和等于目标数。返回这两个数的序号。这个问题可以用hash table做，让给出做所有数字的值作为key，序号作为value。遍历所有数字，如果当前hash table不存在当前数字key，则插入到hash table中。如果hash table中存在 target-当前数字 的key，那么我们就找到了那两个数字。这里的巧妙之处是把求和等于target的过程放在 hash table的key中。需要注意的是找到的两个数字的序号一定是有一个更大的序号， target-当前数字找到的序号更小。

class Solution {public:    vector<int> twoSum(vector<int>& nums, int target)     {          int i,temp;          vector<int> results;          unordered_map<int,int> hm;        for(i=0; i<nums.size(); i++)        {                unordered_map<int ,int>::iterator it;            it=hm.find(nums[i]);            if(it==hm.end())                hm.insert(pair<int, int>(nums[i], i+1));            it=hm.find(target-nums[i]);            if(it!=hm.end())            {                if(it->second<i+1)              {              results.push_back(it->second);                results.push_back(i+1);                return results;                }            }         }          return results;      }};