167. （Two Sum II - Input array is sorted）两数之和 II - 输入有序数组

Posted 2021-09-08 OIqng

• • • 题目：
    • 解题思路：
    • Python代码
    • Java代码
    • C++代码
    • 复杂度分析

题目：

Given an 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.

Return the indices of the two numbers (1-indexed) as an integer array answer of size 2, where 1 <= answer[0] < answer[1] <= numbers.length.

The tests are generated such that there is exactly one solution. You may not use the same element twice.

给定一个已按照非递减（升序）排列 的整数数组 numbers ，请你从数组中找出两个数满足相加之和等于目标数 target 。

返回两个数字(1索引)的索引为大小为2的整数数组answer，其中1 <= answer[0] < answer[1] <= numbers.length。

生成的测试只有一个解决方案。同一个元素不能使用两次。

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.

示例 1：

输入：numbers = [2,7,11,15], target = 9
输出：[1,2]
解释：2 与 7 之和等于目标数 9 。因此 index1 = 1, index2 = 2 。

Example 2:

Input: numbers = [2,3,4], target = 6
Output: [1,3]

示例 2：

输入：numbers = [2,3,4], target = 6
输出：[1,3]

Example 3:

Input: numbers = [-1,0], target = -1
Output: [1,2]

示例 3：

输入：numbers = [-1,0], target = -1
输出：[1,2]

Constraints:

• 2 <= numbers.length <= 3 * $10^4$
• -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.

提示：

• 2 <= numbers.length <= 3 * $10^4$
• -1000 <= numbers[i] <= 1000
• numbers 按 递增顺序 排列
• -1000 <= target <= 1000
• 仅存在一个有效答案

解题思路：

方法：双指针

我们使用双指针指向第一个元素和最后一个元素，并把他们的和与目标值target进行比较。

• 当他们相等时，发现唯一解
• 双指针的和小于目标值target时，左指针右移一位
• 双指针的和大于目标值target时，右指针左移一位

Python代码

class Solution:
    def twoSum(self, numbers: List[int], target: int) -> List[int]:
        left, right = 0, len(numbers)- 1
        while left < right:
            if numbers[left] + numbers[right] == target:
                return [left+1, right+1]
            elif numbers[left] + numbers[right] < target:
                left += 1
            else:
                right -= 1
        return null

Java代码

class Solution {
    public int[] twoSum(int[] numbers, int target) {
        int left = 0, right = numbers.length - 1;
        while (left < right) {
            if (numbers[left] + numbers[right] == target) {
                return new int[]{left + 1, right + 1};
            } else if (numbers[left] + numbers[right] < target) {
                ++left;
            } else {
                --right;
            }
        }
        return null;
    }
}

C++代码

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

复杂度分析

时间复杂度：O(n)，其中 n 是数组的长度。

空间复杂度：O(1)。