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mediumDSA

How do you sort an array of 0s, 1s, and 2s?

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01

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Explain the Dutch National Flag algorithm.

arrayssortingdutch-flag
02

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03

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Dutch National Flag: keep low, mid, high pointers. Sweep mid: swap 0s to the front (low), leave 1s, swap 2s to the back (high). One pass partitions all three values in O(n) time, O(1) space.

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04

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Dutch National Flag
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def sort_colors(nums):
    low, mid, high = 0, 0, len(nums) - 1
    while mid <= high:
        if nums[mid] == 0:
            nums[low], nums[mid] = nums[mid], nums[low]
            low += 1; mid += 1
        elif nums[mid] == 1:
            mid += 1
        else:                               # == 2
            nums[mid], nums[high] = nums[high], nums[mid]
            high -= 1                        # do NOT advance mid
    return nums


# --- demo ---
print(sort_colors([2, 0, 2, 1, 1, 0]))   # [0, 0, 1, 1, 2, 2]
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