LeetCode75 Sort Colors

博客围绕LeetCode上的颜色分类问题展开,给定一个包含红、白、蓝三种颜色元素的数组,要求原地排序。常见的是双趟计数排序法,而本题借助快速排序思想,给出了仅用常数空间的单趟算法,并展示了具体的Java代码实现。

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题目:

       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?

本题是借助快速排序的思想进行解题

    public void sortColors(int[] nums) {
        int zero = -1;          // [0...zero] == 0
        int two = nums.length;  // [two...n-1] == 2
        for(int i = 0 ; i < two ; ){
            if(nums[i] == 1)
                i ++;
            else if (nums[i] == 2)
                swap(nums, i, --two);
            else{ // nums[i] == 0
                assert nums[i] == 0;
                swap(nums, ++zero, i++);
            }
        }
    }

    private void swap(int[] nums, int i, int j){
        int t = nums[i];
        nums[i]= nums[j];
        nums[j] = t;
    }

 

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