Sort Colors - Implementation

Here is the implementation using the Dutch National Flag algorithm:

function sortColors(nums):
    low = 0
    mid = 0
    high = nums.length - 1

    while mid <= high:
        if nums[mid] == 0:
            swap(nums, low, mid)
            low += 1
            mid += 1
        else if nums[mid] == 1:
            mid += 1
        else:
            swap(nums, mid, high)
            high -= 1

This runs in $O(n)$ time with $O(1)$ space (excluding input). Each element is visited at most twice.