Two Sum

Problem

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

---

Idea

• Hash Table
• Use a hashtable to record the index for each element in numbers[]
• For each element number[i] check if target-number[i] is in the hashtable
• Must be sure number[i] appear twice in numbers[] when number[i] == target-number[i]
• Sort

---

Solution

---

Special Cases

• [0,4,3,0], target = 0
• [3,2,4], target = 6
（3 must be checked if another 3 is in numbers[])

---