[leetcode] 75. Sort Colors

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?

代码：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int count[3] = {0};
        for(int i = 0; i < nums.size(); i++){
            count[nums[i]] ++;
        }
        int base = 0;
        for(int i = 0; i < 3; i++){
            int j = 0;
            for(; j < count[i]; j++){
                nums[base + j] = i;
            }
            base += j;
        }
    }
};

Python：

class Solution:
    def sortColors(self, nums):
        """
        :type nums: List[int]
        :rtype: void Do not return anything, modify nums in-place instead.
        """
        zero_index = 0
        two_index = len(nums) - 1
        i = 0
        while i <= two_index:
            if nums[i] == 0:
                nums[zero_index], nums[i] = nums[i], nums[zero_index]
                zero_index += 1
                i += 1
            elif nums[i] == 2:
                nums[two_index], nums[i] = nums[i], nums[two_index]
                two_index -= 1
            else:
                i += 1