75.Sort-Colors

75. Sort Colors

题目地址

https://www.lintcode.com/en/problem/sort-colors/

https://leetcode.com/problems/sort-colors/

题目描述

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?

代码

Approach #1 One pass

Dutch National Flag Problem

public class Solution {
    public void sortColors(int arr) {
    if (arr == null || arr.length <= 1) return;

    // pl 记录 0, 中间都是 1， pr 记录 2
    int pl = 0;
    int pr = arr.length - 1;
    int curr = 0;
    while (curr <= pr) {
      if (arr[curr] == 0) { 
        swap(arr, pl, i);
        pl++;
        curr++;
      } else if (arr[curr] == 1) {
        curr++;
      } else { // arr[curr] == 2
        swap(arr, pr, curr);
        pr--;
      }
    }
  }

  private void swap(int[] arr, int i, int j) {
    int tmp = arr[i];
    arr[i] = arr[j];
    arr[j] = tmp;
  }
}