# 0457. Circular Array Loop

https://leetcode.com/problems/circular-array-loop

## Description

You are playing a game involving a **circular** array of non-zero integers `nums`. Each `nums[i]` denotes the number of indices forward/backward you must move if you are located at index `i`:

* If `nums[i]` is positive, move `nums[i]` steps **forward**, and
* If `nums[i]` is negative, move `nums[i]` steps **backward**.

Since the array is **circular**, you may assume that moving forward from the last element puts you on the first element, and moving backwards from the first element puts you on the last element.

A **cycle** in the array consists of a sequence of indices `seq` of length `k` where:

* Following the movement rules above results in the repeating index sequence `seq[0] -> seq[1] -> ... -> seq[k - 1] -> seq[0] -> ...`
* Every `nums[seq[j]]` is either **all positive** or **all negative**.
* `k > 1`

Return `true` _if there is a **cycle** in_ `nums`_, or_ `false` _otherwise_.

**Example 1:**

```
**Input:** nums = [2,-1,1,2,2]
**Output:** true
**Explanation:**
There is a cycle from index 0 -> 2 -> 3 -> 0 -> ...
The cycle's length is 3.
```

**Example 2:**

```
**Input:** nums = [-1,2]
**Output:** false
**Explanation:**
The sequence from index 1 -> 1 -> 1 -> ... is not a cycle because the sequence's length is 1.
By definition the sequence's length must be strictly greater than 1 to be a cycle.
```

**Example 3:**

```
**Input:** nums = [-2,1,-1,-2,-2]
**Output:** false
**Explanation:**
The sequence from index 1 -> 2 -> 1 -> ... is not a cycle because nums[1] is positive, but nums[2] is negative.
Every nums[seq[j]] must be either all positive or all negative.
```

**Constraints:**

* `1 <= nums.length <= 5000`
* `-1000 <= nums[i] <= 1000`
* `nums[i] != 0`

**Follow up:** Could you solve it in `O(n)` time complexity and `O(1)` extra space complexity?

## ac

```java
```
