[leetcode] 1. Two Sum

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

这道题让找出数组中相加和是target的两个元素的位置，难度为Medium。

最基本的方法是用两层循环遍历数组，如果二者之和是target就找到了这两个元素，不过Online Judge会判超时。

另一种直观的方法是用HashTable，遍历数组，如果target和当前元素的差不在HashTable中将当前元素存入HashTable，如果在HashTable中就找到了这两个元素。具体代码：

class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
vector<int> index;
unordered_map<int, int> hash;

for(int i=0; i<nums.size(); ++i) {
auto it = hash.find(target-nums[i]);
if(it != hash.end()) {
index.push_back(min(i, it->second)+1);
index.push_back(max(i, it->second)+1);
return index;
}
else hash[nums[i]] = i;
}
}
};