LeetCode-Sort Colors

LeetCode-Sort Colors

 

Question information:

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?

Solution: Use two pointers, pointA, to represent 0 and pointB, to represent 2. Use a for loop for search. (1) When 0 is encountered, it is stored directly to the pointA position. (2) In case of 2, make a judgment. 1) is the position of vpointB 0? 2) is the position of pointB 2? 3) is the position of pointB exactly equal to the position of the search, handle these three cases. Finally, assign a value of 1 to for [pointA, pointB.

 

class Solution {public:    void sortColors(int A[], int n) {        int pointA = 0;        int pointB = n-1;        for (int i = 0; i <= pointB; i++) {            if (A[i] == 0)                A[pointA++] = 0;            else if (A[i] == 2) {                   if (A[pointB] == 0) {                       A[pointA++] = 0 ;                       A[pointB--] = 2;                   } else if(A[pointB] == 2) {                     if(i == pointB) {                       pointB--;                     } else {                       while(A[pointB] == 2) {                         pointB--;                         if(i == pointB)                         {                           break;                         }                       }                       if(A[pointB]== 0) {                         A[pointA++] = 0 ;                         A[pointB--] = 2;                       } else {                         A[pointB--] = 2;                       }                     }                   } else A[pointB--] = 2;            }        }        for(int i = pointA;i <= pointB;i++)            A[i] = 1;    }};