Two Sum II - Input Array Is Sorted

mediumTests passed 2026-11-11v1.0.0

TC: O(N)|SC: O(1)

Two Pointers Original Statement (leetcode).md

1. Core Invariant / The Trick

Sorted order guarantees monotonic sum adjustments: if `nums[l] + nums[r] > target`, only decreasing `r` can lower the sum; if `< target`, increment `l`. Return 1-based indices.

2. Complexity Bounds & Proof

Eliminates one row or column of the candidate matrix per step.

3. Minimal Idiomatic Code

def twoSumSorted(numbers: list[int], target: int) -> list[int]:
    l, r = 0, len(numbers) - 1
    while l < r:
        s = numbers[l] + numbers[r]
        if s == target: return [l + 1, r + 1]
        if s < target: l += 1
        else: r -= 1
    return []

4. Failure Modes & Interview Follow-ups

Pitfalls:- Problem requires 1-based indexing `[l + 1, r + 1]`.

Follow-ups:- 3Sum freezes one pointer and calls this 2-pointer reduction.

Personal Notes & Scratchpad (Local-First Yjs CRDT)

Saved instantaneously in IndexedDB. Streams live across peers via Hocuspocus when signed in.

Related Invariants

Two Sum (O(N))3Sum (O(N^2))

Browser Scratchpad

Console Output

Click Run or press Cmd/Ctrl+Enter to execute locally in a Web Worker.