2Sum 2 – Input array is sorted

Problem

This problem is similar to 2Sum Problem, only difference is input array is now sorted.

Detailed problem

Given a 1-indexed array of integers numbers that is already sorted in non-decreasing order, find two numbers such that they add up to a specific target number. Let these two numbers be numbers[index1] and numbers[index2] where 1 <= index1 < index2 <= numbers.length.

Return the indices of the two numbers, index1 and index2, added by one as an integer array [index1, index2] of length 2.

The tests are generated such that there is exactly one solution. You may not use the same element twice.

Your solution must use only constant extra space.

Examples

Example 1:

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. We return [1, 2].

Example 2:

Input: numbers = [2,3,4], target = 6
Output: [1,3]
Explanation: The sum of 2 and 4 is 6. Therefore index1 = 1, index2 = 3. We return [1, 3].

Example 3:

Input: numbers = [-1,0], target = -1
Output: [1,2]
Explanation: The sum of -1 and 0 is -1. Therefore index1 = 1, index2 = 2. We return [1, 2].

Solution

Method 1 - Two Pointer technique

To solve this problem, we can use two points to scan the array from both sides. This is very similar to 2Sum Problem#Method 2B - Using Sorting and 2 Pointer Approach.

Code

Java

public int[] twoSum(int[] numbers, int target) {
	if (numbers == null || numbers.length == 0)
		return null;

	int left = 0;
	int right = numbers.length - 1;

	while (left<right) {
		int currSum = numbers[left] + numbers[right];
		if (currSum<target) {
			++left;
		} else if (currSum > target) {
			right--;
		} else {
			return new int[] {
				left + 1, right + 1
			};
		}
	}

	return null;
}