LeetCode 3819: Rotate Non Negative Elements ·

Table of Contents

Meta Data
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Difficulty: medium First Attempt: 2026-01-25

Submissions: Runtime Error → Accepted

Approach
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Negatives stay put, so only handle non-negatives (Array Template 5): record their positions and values, rotate the values left by k % m (m non-negatives), and fill the positions back in order.

My Attempts
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Attempt 1 did not handle “no non-negative values”: k % len(non_neg) took a modulo by 0. For nums = [-1], k = 2 it raised ZeroDivisionError. Adding the special case passed.

Solution
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class Solution:
    def rotateElements(self, nums: List[int], k: int) -> List[int]:
        idx = [i for i, x in enumerate(nums) if x >= 0]
        if not idx:
            return nums
        vals = [nums[i] for i in idx]
        k %= len(vals)
        vals = vals[k:] + vals[:k]                 # 左旋 k 格
        res = nums[:]
        for i, v in zip(idx, vals):
            res[i] = v
        return res

Complexity
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Time O(n), space O(n).

Mistakes to Avoid
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Make sure the number of non-negatives is not 0 before taking a modulo.
Rotating left is vals[k:] + vals[:k].

Interview Explanation
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I extract the non-negative positions and values, rotate the values left by k, and put them back in order while negatives stay. With no non-negatives I return the array as is. Time O(n).