LeetCode-Sort Colors

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Description：
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?

题意：用0,1,2分别表示红、绿、蓝三种颜色；现给定一个包含元素0,1,2的数组，要求对数组按照红、绿、蓝的顺序进行排序（即数组首部为元素0，中间为元素1，尾部为元素2）；并且要求空间复杂度为O(1)；

解法：假设此时我们要分类的元素只有两类，那么我们可以同时遍历数组的首尾部分进行交换（假设要排序的是元素0和1，那么将尾部的1交换到首部，将首部的0交换到尾部）；同理，对于分类排序的元素有三类时，我们只要保证所有的0和2分别在首部和尾部即可，这样排序之后1自然而然的就处于中间位置；

Java

class Solution {
    public void sortColors(int[] nums) {
        int left = 0;
        int right = nums.length - 1;
        boolean changed = true;
        
        while (changed) {
            changed = false;
            for (int i = left; i <= right; i++) {
                if (nums[i] == 0) {
                    changed = true;
                    exchange(nums, left, i);
                    left++;
                    break;
                }
            }
            for (int i = right; i >= left; i--) {
                if (nums[i] == 2) {
                    changed = true;
                    exchange(nums, right, i);
                    right--;
                    break;
                }
            }
        }
    }
    
    private void exchange(int[] nums, int pos1, int pos2) {
        int temp = nums[pos1];
        nums[pos1] = nums[pos2];
        nums[pos2] = temp;
    }
}