Leetcode(75) 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:

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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?

解法

可以采用counting sort的方法，遍历一遍数组，记录0，1，2的个数，然后按照个数覆写数组。

但题目要求一次遍历，采用类似于快速排序的双指针法，一个指向下一个0的位置，一个指向下一个1的位置，然后进行遍历交换。

具体代码如下：

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class Solution {
    public void sortColors(int[] nums) {
        int i = 0;
        int j = nums.length - 1;
        for (int k = 0; k < nums.length; ) {
            if (k < j && nums[k] > 1) {
                int tmp = nums[k];
                nums[k] = nums[j];
                nums[j] = tmp;
                j--;
            } else if (k > i && nums[k] < 1) {
                int tmp = nums[k];
                nums[k] = nums[i];
                nums[i] = tmp;
                i++;
            } else {
                k++;
            }
        }
    }
}