> For the complete documentation index, see [llms.txt](https://jaywin.gitbook.io/leetcode/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jaywin.gitbook.io/leetcode/solutions/1562-find-latest-group-of-size-m.md).

# 1562. Find Latest Group of Size M

<https://leetcode.com/problems/find-latest-group-of-size-m>

## Description

Given an array `arr` that represents a permutation of numbers from `1` to `n`. You have a binary string of size `n` that initially has all its bits set to zero.

At each step `i` (assuming both the binary string and `arr` are 1-indexed) from `1` to `n`, the bit at position `arr[i]` is set to `1`. You are given an integer `m` and you need to find the latest step at which there exists a group of ones of length `m`. A group of ones is a contiguous substring of 1s such that it cannot be extended in either direction.

Return *the latest step at which there exists a group of ones of length **exactly*** `m`. *If no such group exists, return* `-1`.

**Example 1:**

```
**Input:** arr = [3,5,1,2,4], m = 1
**Output:** 4
**Explanation:**Step 1: "00100", groups: ["1"]
Step 2: "00101", groups: ["1", "1"]
Step 3: "10101", groups: ["1", "1", "1"]
Step 4: "11101", groups: ["111", "1"]
Step 5: "11111", groups: ["11111"]
The latest step at which there exists a group of size 1 is step 4.
```

**Example 2:**

```
**Input:** arr = [3,1,5,4,2], m = 2
**Output:** -1
**Explanation:**Step 1: "00100", groups: ["1"]
Step 2: "10100", groups: ["1", "1"]
Step 3: "10101", groups: ["1", "1", "1"]
Step 4: "10111", groups: ["1", "111"]
Step 5: "11111", groups: ["11111"]
No group of size 2 exists during any step.
```

**Example 3:**

```
**Input:** arr = [1], m = 1
**Output:** 1
```

**Example 4:**

```
**Input:** arr = [2,1], m = 2
**Output:** 2
```

**Constraints:**

* `n == arr.length`
* `1 <= n <= 10^5`
* `1 <= arr[i] <= n`
* All integers in `arr` are **distinct**.
* `1 <= m <= arr.length`

## ac

```java
```