Two Sum II - Input Array Is Sorted — LeetCode 167 Python Solution

The problem

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.

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. We return [1, 2].

Python solution

class Solution:
    def twoSum(self, numbers: List[int], target: int) -> List[int]:
        n = len(numbers)
        for i in range(n - 1):
            x = target - numbers[i]
            j = bisect_left(numbers, x, lo=i + 1)
            if j < n and numbers[j] == x:
                return [i + 1, j + 1]

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 167. Two Sum II - Input Array Is Sorted is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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