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：这道题题目没有给出numbers是已排序的，所以我们不能直接使用从头尾两个开始同时搜索。应该换一种思路——哈希表，可以使用map<int,int>记录numbers每个元素的位置+1信息。然后循环遍历numbers，找出(target-numbers[i])，满足要求则保存位置信息。注意这里还要要求index1必须小于index2的位置索引信息。

class Solution {
public:
    vector<int> twoSum(vector<int> &numbers, int target) {
        vector<int> result(2,0);
        int n=numbers.size();
        if(n<=1)
            return result;
        map<int,int> data;
        map<int,int>::iterator iter;
        for(int i=0;i<n;i++)
        {
            iter=data.find(target-numbers[i]);
            if(iter!=data.end())
            {
                result[0]=iter->second;//这里为什么没有判断那个在前呢
                result[1]=i+1;
            }
            else
            {
                data[numbers[i]]=i+1;
            }
        }
        return result;
    }
};