75. Sort Colors

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?

---
Time - O(n)
Space - O(1)
---

	class Solution {
	public void sortColors(int[] nums) {
	int r = 0, w = 0, b = nums.length - 1;
	while (w <= b) {
	if (nums[w] == 0) {
	// Move 0's to beginning
	nums[w] = nums[r];
	nums[r] = 0;
	r++;
	w++;
	} else if (nums[w] == 2) {
	// Move 2's to end
	nums[w] = nums[b];
	nums[b] = 2;
	b--;
	} else {
	w++;
	}
	}
	}
	}

view raw 75.SortColorsThreePointers.java hosted with ❤ by GitHub

	class Solution {
	public void sortColors(int[] nums) {
	int r = 0, w = 0, b = 0;
	for (int i = 0; i < nums.length; i++) {
	switch (nums[i]) {
	case 0:
	r++;
	break;
	case 1:
	w++;
	break;
	case 2:
	b++;
	}
	}
	for (int i = 0; i < nums.length; i++) {
	if (r > 0) {
	nums[i] = 0;
	r--;
	continue;
	}
	if (w > 0) {
	nums[i] = 1;
	w--;
	continue;
	}
	nums[i] = 2;
	}
	}
	}

view raw 75.SortColorsTwoPassCountingSort.java hosted with ❤ by GitHub