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How do you rotate an array by k positions?

Q: How do you rotate an array by k positions?

**Intuition.** Rotating right by k means the last k elements wrap to the front. The elegant O(1)-space trick is the **reversal method**: reverse the whole array, then reverse the first k and the remaining n−k . Two flips put everything in place. [1,2,3,4,5], k=2 → [4,5,1,2,3] 1) reverse all: 5 4 3 2 1 <text x="20" y="78" fill="currentColor" opacity="0.7" text-anchor="star

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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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