Two Sum II - Input array is sorted LT167

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.

Idea 1. two pointer scan

Time complexity: O(n)

Space complexity: O(1)

Idea 2. hashMap

Time Complexity: O(n)

Space complexity: O(n)

Idea 3. binary Search

Time Complexity: O(nlgn)

Space complexity: O(n)

 1 class Solution {
 2     public int[] twoSum(int[] numbers, int target) {
 3         for(int left = 0, right = numbers.length-1; left < right; ) {
 4             int sum = numbers[left] + numbers[right];
 5             if(sum == target) {
 6                 return new int[] {left + 1, right + 1};
 7             }
 8             else if(sum < target) {
 9                 ++left;
10             }
11             else {
12                --right;
13             }
14         }
15         
16         return new int[]{0, 0};
17     }
18 }