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mediumDSA

How do you rotate an array by k positions?

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01

Understand the problem

Explain array rotation.

arraysrotation
02

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03

Study the solution

The reversal trick: reverse the whole array, then reverse the first k and the remaining n−k. This rotates in O(n) time, O(1) space, avoiding extra arrays or repeated single-step shifts.

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04

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Reversal method (O(1) space)
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def rotate(nums, k):
    n = len(nums)
    k %= n
    def reverse(lo, hi):
        while lo < hi:
            nums[lo], nums[hi] = nums[hi], nums[lo]
            lo += 1; hi -= 1
    reverse(0, n - 1)
    reverse(0, k - 1)
    reverse(k, n - 1)
    return nums


# --- demo ---
print(rotate([1, 2, 3, 4, 5], 2))   # [4, 5, 1, 2, 3]
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