LeetCode Daily Question 14

3 月 19 日 每日一题

题目：167. Two Sum II - Input Array Is Sorted

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.

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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].

思路

这道题可以说是 15. 3Sum 的弱化版，思路很简单（竟然是道 medium），双指针移动，找到 total = target 时 return。

代码

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class Solution {
public:
    vector<int> twoSum(vector<int>& numbers, int target) {
        vector<int> res;
        int left = 0;
        int right = int(numbers.size()) - 1;
        while(left < right){
            int total = numbers[left] + numbers[right];
            if(total < target)
                left++;
            else if(total > target)
                right--;
            else if(total == target){
                res.push_back(left + 1);
                res.push_back(right + 1);
                break;
            }
        }
        return res;
    }
};