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
如果采用暴力搜索的话，是O(n^2)的世界复杂度。可以考虑把目标数字和现有集合S中的数字想减，得到一个辅助数组A。然后依次取取A中的元素，从现有几何S中找相同元素，如果有，则返回此数字的index。 当然，这种方法的复杂度依然是O(n^2). 此时我们可以考虑先使用排序算法（快排等）把集合S中的数字进行排序（注意要保存数字的下标）。对于排好序的数组，我们就可以使用各种查找算法来加速了。
上面是我的初步想法，后来发现可以通过构建哈希表，使得查找速度变为O(e)。不过哈希表对空间的消费比较大。
Add Two Numbers:
You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
此题主要是要考虑到指针操作，以及两数相加的各种情况。算法的东西没有考到。