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

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

**Intuition.** With only three values you don't need a comparison sort. The **Dutch National Flag** algorithm makes one pass with three pointers — low , mid , high — partitioning the array into 0s, 1s, and 2s in place. low region = 0s, mid scans, high region = 2s 0 <text x="134" y="69" fill="

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Explain the Dutch National Flag algorithm.

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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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