LeetCode 167 - Two Sum II - Input array is sorted

• Difficulty: Easy.
• Related Topics: Array, Two Pointers, Binary Search.
• Link: leetcode

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 1

/**
 * @param {number[]} numbers
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function (numbers, target) {
  let startPoint = 0;
  let endPoint = numbers.length - 1;
  for (i = 0; i < numbers.length; i++) {
    if (target !== numbers[startPoint] + numbers[endPoint]) {
      target > numbers[startPoint] + numbers[endPoint]
        ? startPoint++
        : endPoint--;
    }
  }
  return [startPoint + 1, endPoint + 1];
};

First thought

var twoSum = function (numbers, target) {
  if (numbers.length === 2) {
    return [1, 2];
  }

  let sum = 0;
  let left = 0;
  let right = numbers.length - 1;

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

Complexity:

• Time complexity : O(n).
• Space complexity : O(1).