알고리즘 읽기 노트

Binary Search

Binary search finds a target in a sorted search space by repeatedly discarding half of the remaining range.

유형알고리즘카테고리Search깊이초급일반적인 복잡도O(log n)

짧은 면접 답변

이 멘탈 모델부터 시작하세요

Binary search finds a target in a sorted search space by repeatedly discarding half of the remaining range.

간단한 설명

불필요한 전문용어 없이 동작 방식 이해하기

Compare target with the middle element.

If target is smaller, continue in the left half; if larger, continue in the right half.

실행

알고리즘을 연속된 의사결정으로 읽기

Set left and right bounds.

Compute mid without leaving the range.

Move exactly one bound after each comparison until found or left > right.

기억해 둘 작은 예제

Searching 23 in [3, 8, 12, 23, 31, 44] checks the middle, then keeps only the half where 23 can still exist.

읽기 쉬운 JavaScript 구현JavaScript

function binarySearch(items, target) {
  let left = 0;
  let right = items.length - 1;

  while (left <= right) {
    const mid = left + Math.floor((right - left) / 2);
    if (items[mid] === target) return mid;
    if (items[mid] < target) left = mid + 1;
    else right = mid - 1;
  }
  return -1;
}

이 코드를 읽는 방법

문법을 외우지 마세요. 각 줄에서 바뀌는 상태를 따라가고 위에서 설명한 불변식과 연결하세요.

복잡도

O(log n)

Do not quote this complexity mechanically. Ask which operation or input dimension it describes, then derive the cost from the loops, recursion depth, data-structure operations and any preprocessing assumptions.

언제 사용하는가

적합한 경우

Match the algorithm to the structure and preprocessing state of the data.

흔한 실수

보통 어디에서 문제가 생기는가

Using it on unsorted data.
Off-by-one errors and infinite loops.
Not defining whether boundaries are inclusive or half-open.

면접에서 설명하는 방법

전문적인 답변 구조

Say the invariant: if the target exists, it is inside the current search interval. Then explain how each comparison preserves that invariant while halving the interval.

문제→핵심 아이디어→불변식 / 상태→Big-O→트레이드오프

빠른 면접 체크

What problem does Binary Search solve?

Binary search finds a target in a sorted search space by repeatedly discarding half of the remaining range.

복잡도에 대해 무엇을 언급해야 하나요?

Its typical cost here is O(log n), but a complete answer should connect that cost to the representation, input constraints and the dominant operations.

가장 흔한 실수는 무엇인가요?

Using it on unsorted data.

계속 읽기Sorted arraysLower/upper boundBinary search on answer

function binarySearch(items, target) { let left = 0; let right = items.length - 1; while (left <= right) { const mid = left + Math.floor((right - left) / 2); if (items[mid] === target) return mid; if (items[mid] < target) left = mid + 1; else right = mid - 1; } return -1; }