75. Sort Colors (Medium)

https://leetcode.com/problems/sort-colors/

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.

Example:

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?

Hints

Solutions

class Solution {
    public void sortColors(int[] nums) {
        int i = 0;
        int j = nums.length - 1;
        int k = 0;
        while (k <= j) {
            if (nums[k] == 0) {
                swap(nums, i, k);
                i++;
            } else if (nums[k] == 2) {
                swap(nums, j, k);
                j--;
                k--;
            }
            k++;
        }
    }
    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}
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