LeetCode #167 Two Sum II — Input Array Is Sorted— Swift solution

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Description:

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 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].

Constraints:

• 2 <= numbers.length <= 3 * 104
• -1000 <= numbers[i] <= 1000
• numbers is sorted in non-decreasing order.
• -1000 <= target <= 1000
• The tests are generated such that there is exactly one solution.

Step by step:

This solution uses a technique called “two-pointer”. Here sums up some patterns of two pointers and gives you an idea of what two pointers is.

Basically, we can set a left pointer at the beginning and a right one at the end, which allow us to track the indices of the numbers array.

Because the given array is sorted and ascending, we then can add up the two numbers, which result in 3 types of scenarios.

1. sum > target: shift the right pointer to the left
2. sum < target: shift the left pointer to the right
3. sum == target: BINGO, return the answer

Code:

That’s it for now, see ya next time!

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