#167. Two Sum II - Input Array Is Sorted

• Solved

Description

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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.

My Solution

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public class Solution {
    public int[] TwoSum(int[] numbers, int target) {
        for(int i = 0; i < numbers.Length - 1; i++) {
            for(int j = i + 1; j < numbers.Length; j++) {
                if(numbers[i] + numbers[j] == target) {
                    return [i + 1, j + 1];
                }
            }
        }

        return [];
    }
}

Runtime

343 ms / Beats 6.39%

Memory

49.59 MB / Beats 92.65%

Big O Notation

Time complexity: O(n^2)
Space complexity: O(1)

Best Solution

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public class Solution {
    public int[] TwoSum(int[] numbers, int target) {
        int left = 0;
        int right = numbers.Length - 1;

        while (left < right) {
            int total = numbers[left] + numbers[right];

            if (total == target) {
                return [left + 1, right + 1];
            } else if (total > target) {
                right--;
            } else {
                left++;
            }
        }

        return [];
    }
}

Runtime

0 ms / Beats 100%

Memory

49.81 MB / Beats 54.73%

Big O Notation

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