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

　　建立哈希表，遍历数组，对于每一个元素n，首先看它是否存在于哈希表中，否则将{target-n,index}存放于一个哈希表中，是则得到解index1=hash(n),index2=index;

　　c++实现：

vector<int> twoSum(vector<int>& nums, int target) {
        map<int,int> m;
        vector<int> ret;
        for(int i=0;i<nums.size();++i){
            if(m.find(nums[i])!=m.end()){
                ret.push_back(m[nums[i]]);
                ret.push_back(i+1);
                break;
            }
            else{
                m.insert({target-nums[i],i+1});
            }
        }
        return ret;
    }

　　java实现：

public int[] twoSum(int[] nums, int target) {
        Hashtable<Integer,Integer> m = new Hashtable<Integer,Integer>();
        int[] ret = new int[2];
        
        for(int i=0;i<nums.length;++i){
            if(m.get(nums[i])==null){
                m.put(target-nums[i],i+1);
            }
            else{
                ret[0] = m.get(nums[i]);
                ret[1] = i+1;
                break;
            }
        }
        return ret;
    }