Circular Array Loop — LeetCode 457 Python Solution

The problem

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 abs(nums[i]) steps backward.

Example

Input

nums = [2,-1,1,2,2]

Output

true

Explanation

The graph shows how the indices are connected. White nodes are jumping forward, while red is jumping backward.

Python solution

class Solution:
    def circularArrayLoop(self, nums: List[int]) -> bool:
        n = len(nums)

        def next(i):
            return (i + nums[i] % n + n) % n

        for i in range(n):
            if nums[i] == 0:
                continue
            slow, fast = i, next(i)
            while nums[slow] * nums[fast] > 0 and nums[slow] * nums[next(fast)] > 0:
                if slow == fast:
                    if slow != next(slow):
                        return True
                    break
                slow, fast = next(slow), next(next(fast))
            j = i
            while nums[j] * nums[next(j)] > 0:
                nums[j] = 0
                j = next(j)
        return False

Complexity

Pattern: Two Pointers

Use the order already in the input to discard half the search space at every step. LeetCode 457. Circular Array Loop is filed here because LeetCode tags it Two Pointers, which is the vocabulary this hub collects.

The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.

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