075. Sort Colors

At a Glance

• Topic: Array
• Pattern: Analyze Pattern
• Difficulty: Medium
• LeetCode: 075

Problem Statement

Given an array nums with n objects colored red, white, or blue, sort them in-place so that objects of the same color are adjacent, with the colors in the order red, white, and blue.

We will use the integers 0, 1, and 2 to represent the color red, white, and blue, respectively.

You must solve this problem without using the library's sort function.

Example 1:

Input: nums = [2,0,2,1,1,0] Output: [0,0,1,1,2,2]

Example 2:

Input: nums = [2,0,1] Output: [0,1,2]

Constraints:

n == nums.length
1 <= n <= 300
nums[i] is either 0, 1, or 2.

Follow up: Could you come up with a one-pass algorithm using only constant extra space?

Approach & Solution Steps

• Counting values is two passes. Built-in sort is O(n log n). Dutch National Flag is one pass and constant space.

Optimal JS Solution

function sortColors(nums) {
	let low = 0, mid = 0, high = nums.length - 1;
	while (mid <= high) {
		if (nums[mid] === 0) {
			[nums[low], nums[mid]] = [nums[mid], nums[low]];
			low++; mid++;
		} else if (nums[mid] === 1) mid++;
		else {
			[nums[mid], nums[high]] = [nums[high], nums[mid]];
			high--;
		}
	}
}

Edge Cases & Pitfalls

• Always consider empty or null inputs.
• Watch out for off-by-one index errors.