#11 Medium Time O(n) Space O(1) Animated

Two Sum II - Input Array Is Sorted

Find two numbers in a sorted array that add up to target.

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Pattern Two pointers shrink the search space

Use ordering or symmetry to decide which boundary can move without discarding a valid answer.

Approach: Two Pointers — Sorted Input

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Core Insight

Use ordering or symmetry to decide which boundary can move without discarding a valid answer.

Common Pitfall

Move the pointer justified by the comparison; moving both can skip a valid pair.

Description

Find two numbers in a sorted array that add up to target.

Constraints

Time Complexity: O(n)
Space Complexity: O(1)

Solution — Two Pointers — Sorted Input

TimeO(n) SpaceO(1)

Implementation Plan

Translate the invariant into these small, testable moves.

Place pointers at the relevant boundaries.
Evaluate the pair or window.
Use the comparison to move exactly one boundary.
Stop when the pointers cross or the target is found.

def twoSum(numbers, target):
    left, right = 0, len(numbers) - 1
    while left < right:
        total = numbers[left] + numbers[right]
        if total == target:
            return [left + 1, right + 1]
        if total < target:
            left += 1
        else:
            right -= 1

Valid Palindrome

3Sum

Two Sum II - Input Array Is Sorted

Steps

Description

Constraints

Tags

Solution — Two Pointers — Sorted Input