Leetcode Algorithms

167. Two Sum II — Input array is sorted

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 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.

Example 2:

Input: numbers = [2,3,4], target = 6
Output: [1,3]

Example 3:

Input: numbers = [-1,0], target = -1
Output: [1,2]

Constraints:

• 2 <= nums.length <= 3 * 104
• -1000 <= nums[i] <= 1000
• nums is sorted in increasing order.
• -1000 <= target <= 1000

Solution:

/**
 * @param {number[]} numbers
 * @param {number} target
 * @return {number[]}
 */
let twoSum = function(numbers, target){
    let leftPointer = 0;
    let rightPointer = numbers.length - 1;
    let result = [];
    while(leftPointer < rightPointer){
        let leftElement = numbers[leftPointer];
        let rightElement = numbers[rightPointer];
        if(leftElement + rightElement === target){
            // needs to return indices
            result.push(leftPointer + 1);
            result.push(rightPointer + 1);
            break;
        }else if(leftElement + rightElement > target){
            rightPointer --;
        } else{
            leftPointer ++;       
        }
    }
  return result;
}

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