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
求目标整数的两个加数，并返回其下标（C++下标+1）。
题目不难，值得注意的就是上面黄字的地方。因为可以利用这个条件稍微优化一点，使得循环由n^2
编程n^2/2。循环少一半。
class Solution {
public:
vector twoSum(vector &numbers, int target) {
vector twoIndices;
if(numbers.empty()) return twoIndices;
for(int i=0; i