leetcode/islands/decks/binary-search.md

## Binary Search

Halve a **monotonic** search space each step → O(log n).

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## Recognition

- Sorted input + "find / insert position"
- "Minimize the maximum" / "first value where condition flips"
- Answer itself is numeric and checkable → binary search **on the answer**

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## Template

```js
function search(nums, target) {
  let lo = 0;
  let hi = nums.length - 1;
  while (lo <= hi) {
    const mid = lo + ((hi - lo) >> 1);
    if (nums[mid] === target) return mid;
    if (nums[mid] < target) lo = mid + 1;
    else hi = mid - 1;
  }
  return -1; // lo is the insert position
}
```

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## First-true boundary

```js
// smallest index where ok(i) is true; ok is false...false true...true
let lo = 0;
let hi = n; // exclusive; n means "never true"
while (lo < hi) {
  const mid = lo + ((hi - lo) >> 1);
  if (ok(mid)) hi = mid;
  else lo = mid + 1;
}
return lo;
```

Most "hard" binary searches are this shape in disguise.

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## Pitfalls

- Off-by-one: pick `lo <= hi` **or** `lo < hi` and stay consistent
- `mid = lo + ((hi - lo) >> 1)` avoids overflow and floats
- No progress → infinite loop: every branch must shrink the range

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## Recall

- What property must the search space have? (not "sorted" — *monotonic*)
- Where does `lo` land when the target is absent?
- When do you search the answer space instead of the array?