0075. Sort Colors

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

Description

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]

Example 3:

**Input:** nums = [0]
**Output:** [0]

Example 4:

**Input:** nums = [1]
**Output:** [1]

Constraints:

• n == nums.length

• 1 <= n <= 300

• nums[i] is 0, 1, or 2.

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

ac

// 1.counting sort; 2. two pass quick select; 3. special partition
// important** for loop, go through each loop; while loop, pass some steps. note the difference.

class Solution {
    public void sortColors(int[] nums) {
        // edge cases
        if (nums.length < 2) return;

        // left, right. i walks, if 0 swap to left, if 2 swap to right.
        // while i < right
        int l = 0, r = nums.length - 1;
        for (int i = 0; i <= r; i++) {
            while (nums[i] == 2 && i < r) {
                swap(nums, i, r);
                r--;
            }
            while (nums[i] == 0 && i > l) {
                swap(nums, l, i);
                l++;
            }
        }
    }
    private void swap(int[] nums, int x, int y) {
        int tmp = nums[x];
        nums[x] = nums[y];
        nums[y] = tmp;
    }
}