LeetCode #167 Two Sum II — Input array is sorted

Easy

Problem

Given an array of integers that is already sorted in ascending order, 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.

Note:

• Your returned answers (both index1 and index2) are not zero-based.
• You may assume that each input would have exactly one solution and you may not use the same element twice.

Example:

Input: numbers = [2,7,11,15], target = 9
Output: [1,2]
Explanation: The sum of 2 and 7 is 9. Therefore index1 = 1, index2 = 2.

Solution

Use two pointer head and tail. Sum them up and compare the result with given target.

1. If sum < target: It means we need larger value to enlarge our sum and make it closer to target. Hence, we will move head to next position.
2. If sum > target: It means we have to reduce our selection so it can be closer to target. Therefore, we will move tail to previous position.
3. If sum == target: Done.

Complexity

head and tail will finally meet each other after examining all elements in the list. Therefore, it takes O(n) times if n denotes to length of given list numbers.

And it only needs O(1) extra space.