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

思路：数据结构map，记录剩余值

class Solution {
public:
    vector<int> twoSum(vector<int> &numbers, int target) {
        vector<int> res;
        //check validation;
        if(numbers.empty()) return res;
        
        //check special case or bound;
        size_t n=numbers.size();
        if(n==1) return res;
        
        //general case
        unordered_map<int,int> map;
        
        int rest=0;
        for(int i=0;i<n;i++){
            //check if find the two number
            if(map.count(numbers[i])){
                res.push_back(map[numbers[i]]);
                res.push_back(i+1);
                break;
            }
            //compute the number[i]'s rest
            rest = target-numbers[i];
            map[rest]=i+1;
        }
        return res;
    }
};