167. Two Sum II - Input Array Is Sorted

Given a 1-indexed array of integers sorted in non-decreasing order, find two numbers that add up to target and return their indices [index1, index2] (1-indexed). Use only constant extra space.

Start at both ends. Since the array is sorted, a sum that is too small can only grow by moving l right, and a sum that is too big can only shrink by moving r left.

1function twoSum(numbers: number[], target: number): number[] {
2  let l = 0, r = numbers.length - 1;
3  while (l < r) {
4    const sum = numbers[l] + numbers[r];
5    if (sum === target) return [l + 1, r + 1];
6    if (sum < target) l++;
7    else r--;
8  }
9  return [];
10}