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：暴力想法是两重循环，但是时间开销比较大，比较好的思路是使用哈希表，利用哈希表查找高效的优势，可将算法的时间复杂度降为O(n).

class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
vector<int> results;
map<int,int>hmap;
for(int i = 0; i < nums.size();++i){
if(!hmap.count(nums[i]))
hmap.insert(pair<int,int>(nums[i],i));
if(hmap.count(target-nums[i])){
int n = hmap[target-nums[i]];
if(n<i){//确定两个数是不同的
results.push_back(n+1);
results.push_back(i+1);
return results;
}
}
}
return results;
}
};