167. Two Sum II - Input array is sorted #

题目 #

Given an array of integers that is already sorted in ascending order, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2.

Note:

• Your returned answers (both index1 and index2) are not zero-based.
• You may assume that each input would have exactly one solution and you may not use the same element twice.

Example:

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.

题目大意 #

找出两个数之和等于 target 的两个数字，要求输出它们的下标。注意一个数字不能使用 2 次。下标从小到大输出。假定题目一定有一个解。

解题思路 #

这一题比第 1 题 Two Sum 的问题还要简单，因为这里数组是有序的。可以直接用第一题的解法解决这道题。

代码 #

package leetcode

// 解法一 这一题可以利用数组有序的特性
func twoSum167(numbers []int, target int) []int {
	i, j := 0, len(numbers)-1
	for i < j {
		if numbers[i]+numbers[j] == target {
			return []int{i + 1, j + 1}
		}
		if numbers[i]+numbers[j] < target {
			i++
		} else {
			j--
		}
	}
	return nil
}

// 解法二 不管数组是否有序，空间复杂度比上一种解法要多 O(n)
func twoSum167_1(numbers []int, target int) []int {
	m := make(map[int]int)
	for i := 0; i < len(numbers); i++ {
		another := target - numbers[i]
		if idx, ok := m[another]; ok {
			return []int{idx + 1, i + 1}
		}
		m[numbers[i]] = i
	}
	return nil
}