Brooke

一、題目

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.

 

二、程式作法

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

        while (left < right)
        {
            if (numbers[left] + numbers[right] == target)
                return new int[] { left + 1, right + 1 };

            if (numbers[left] + numbers[right] < target)
                left++;
            else
                right--;
        }

        return new int[] { };
    }
}

 

三、思路筆記

題目要的結果是找出陣列中的兩元素使加起來等於 target 值。

那我們就宣告左右兩指標，代表陣列的初始與結尾指標，

我們利用這左右兩指標來向陣列中間逼近答案。