Sort Colors -- leetcode

Sort Colors -- leetcode

 

Given an array with n objects colored red, white or blue, sort them 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.

Click to show follow up.

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.

Cocould you come up with an one-pass algorithm using only constant space?

 

Algorithm 1: red, white, and blue pointers move forward and backward

This algorithm can be understood by flushing the wall.

Assume there are x red, y White, and z blue.

First, fl all the blue (x + y + z ).

Then the image is white from the head brush (x + y.

Then, brush x from the head to red.

 

Class Solution {public: void sortColors (int A [], int n) {int red =-1, white =-1, blue =-1; for (int I = 0; I
 
  
 
  
 
Algorithm reference:
  
Https://leetcode.com/discuss/1827/anyone-with-one-pass-and-constant-space-solution
  
Algorithm 2: red, white, and two pointers move from front to back, and the blue pointer moves from back to back.
  
 
  
class Solution {public:    void sortColors(int A[], int n) {        int red = -1, white = 0, blue = n;        while (white < blue) {            if (A[white] == 1) {                ++white;            }            else if (A[white] == 0) {                A[white++] = 1;                A[++red] = 0;            }            else {                A[white] = A[--blue];                A[blue] = 2;            }        }    }};