2Sum I

来源：Leetcode

原帖：http://oj.leetcode.com/problems/two-sum/

题目：
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

代码：
1] 使用hashtable

 class Solution {  
 public:   
     vector<int> twoSum(vector<int> &numbers, int target) {   
         unordered_map<int, int> map; // <value, index>   
         for (int i = 0; i < numbers.size(); ++i) {  
             int val = target - numbers[i];  
             if (map.count(val)) {  
                 vector<int> res = {map[val], i+1};  
                 return res;  
             } else {  
                 map[numbers[i]] = i + 1;  
             }  
         }  
         return {-1,-1};  
     }  
 };  

2] sorting & two pointers

 bool compare(const pair<int, int> &a, const pair<int, int> &b) {  
     return a.first < b.first;  
 }  
   
 class Solution {  
 public:    
     //time: O(nlogn). pair<val,index> nums  
     vector<int> twoSum(vector<int> &numbers, int target) {   
         vector<pair<int, int> > nums(numbers.size());  
         for (int i = 0; i < numbers.size(); ++i) {  
             nums[i] = {numbers[i], i+1};  
         }  
         sort(nums.begin(), nums.end(), compare); // O(nlogn)  
   
         int l = 0, r = nums.size() - 1;  
         while (l < r) {  
             int sum = nums[l].first + nums[r].first;  
             if (sum == target) {  
                 break;  
             } else if (sum < target) {  
                 l++;  
             } else {  
                 r--;  
             }  
         }  
         vector<int> res;  
         res.push_back(min(nums[l].second, nums[r].second));  
         res.push_back(max(nums[l].second, nums[r].second));  
         return res;  
     }  
 };  

思考：
这道题有很多的变形。Leetcode上这道题有2Sum I, II, III.
其中I只是返回了其中一个解的索引。如果扩展到返回所有的解的组合呢？
Consider duplicates, use hashtable <value, index>, after visiting the number, set index = -1 in hashtable. Otherwise, insert <num, index> into hashtable.