Sort Colors

Problem

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.

Solution

/**
 * @param {number[]} nums
 * @return {void} Do not return anything, modify nums in-place instead.
 */
var sortColors = function(nums) {
    let l = 0; // pointer to end of 0s
    let r = nums.length - 1; // pointers to start of 2s

    for (let i = 0; i <= r; i++) {
        if (nums[i] === 0) {
            [nums[i], nums[l]] = [nums[l], nums[i]];
            l++;
        } else if (nums[i] === 2) {
            [nums[i], nums[r]] = [nums[r], nums[i]];
            r--;
            i--; // might have swapped 0 to index i, so need to check again
        }
    }
};

We will implement a two pointers solution. Our solution is similar to the partition method for quicksort. More specifically, we iterate through nums while moving 0s to index l and 2s to index r. This pushes all 0s to the start, 2s to the end, and 1s to the centre of nums.

Problem​

Solution​

Problem

Solution