75. Sort Colors (Medium)


Given an array 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.

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

Note: You are not suppose to use the library's sort function for this problem.


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

Follow up:

  • A rather straight forward solution is a two-pass algorithm using counting sort.
    First, iterate the array counting number of 0's, 1's, and 2's, then overwrite array with total number of 0's, then 1's and followed by 2's.
  • Could you come up with a one-pass algorithm using only constant space?


class Solution {
    public void sortColors(int[] nums) {
        // slot for 0 starts from 0 and moves forward
        int slotFor0 = 0;

        // slot for 2 starts from len-1 and moves reversely
        int sloptFor2 = nums.length - 1;

        int ptr = 0;

        while (ptr <= sloptFor2) {
            if (nums[ptr] == 0) {
                swap(nums, slotFor0, ptr);

                // Here, the reason why ptr does not step back to check element
                // from slotFor0 is that it already checked since ptr always moves
                // faster than slotFor0


            } else if (nums[ptr] == 2) {
                swap(nums, ptr, sloptFor2);

                // Step back to the newly swapped element from slotFor2 because it has not
                // been checked yet.



    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;

Incorrect Solutions


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