Q167 Two Sum II - Input array is sorted

Two Sum II - Input array is sorted

Question

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.

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.

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.

Approach 1: Dictionary

Intuition and Algorithm

class Solution(object):
    def twoSum(self, numbers, target):
        dic = {}
        for i, num in enumerate(numbers): # 获得编号i(从0开始)和值num
            if target - num in dic:
                return [dic[target - num] + 1, i + 1]
            dic[num] = i


def main():
    numbers = [2, 7, 11, 15]
    target = 9
    solution=Solution().twoSum(numbers,target)
    print(solution)


if __name__ == "__main__":
    main()

Complexity Analysis

• Time Complexity: O(N), where N is the length of numbers.
• Space Complexity: O(1).
• Runtime: 40 ms, faster than 98.01% of Python online submissions for Two Sum II - Input array is sorted.
• Memory Usage: 12.1 MB, less than 40.00% of Python online submissions for Two Sum II - Input array is sorted.