Leetcode 167 - Two sum

Note:

  • A very classical double pointers questions.

Question:

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.

Example:

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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].

Code:

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public class Solution {
public int[] TwoSum(int[] numbers, int target) {
var left = 0;
var right = numbers.Length - 1;
while (left < right) {
var sum = numbers[left]+numbers[right];
if (sum == target) return new int[2]{left+1, right+1};
if (sum < target) {
left++;
} else {
right--;
}
}
return new int[2]{-1,-1};
}
}