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 onlyconstant extra space?

來源：力扣（LeetCode）
連結：https://leetcode-cn.com/problems/sort-colors

class Solution {
    
    // 優化，只遍歷一次
    public void sortColors(int[] nums) {
        int left = 0, right = nums.length - 1;
        int i = left;
        
        while (i <= right) {
            // 2歸位,right指標左移
            if (nums[i] == 2) {
                swap(nums, i, right--);
            } else if (nums[i] == 0) {
                // 0放到最左邊
                swap(nums, i++, left++);
            } else {
                i++;
            }
        }
        
    }
    
    /**
    本質考察快速排序思想
    兩次遍歷：第一次遍歷先將0歸位，第二次遍歷將1歸位，自然2也就歸位了
    */ 
    public void sortColors1(int[] nums) {
        int pos = 0;
        // 將0歸位
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == 0) {
                swap(nums, i, pos);
                pos++;
            }
        }
        
        // 將1歸位
        for (int j = pos; j < nums.length; j++) {
            if (nums[j] == 1) {
                swap(nums, j, pos);
                pos++;
            } 
        }
        
    }
    
    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
    
}