Leetcode 75. Sort Colors 算法

描述
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.
【Leetcode 75. Sort Colors】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.
For example: [1,2,1,1,0,0,2,1,0]
思路

思路1：快速排序的变体，见《算法-第四版》P189
思路2：桶排序
更正一下思路一：这道题其实可能可以不用递归调用的，因为只有三个数，一次运行后，有可能就会对的，但是必须保证切分点是中间值，如原始输入为[1,0,2]可以Accept，[0,1,2]就不能过。所以还是需要递归。

实现
快速排序

class Solution {public void exch(int[] nums, int i, int j){ int temp = nums[i]; nums[i] = nums[j]; nums[j] = temp; }public void sortQuickThreeWay(int[] nums, int lo, int hi){ if (hi <= lo) return; int less = lo; int i = lo + 1; int great = hi; int v = nums[lo]; while(i <= great){ if (nums[i] < v) exch(nums, less++, i++); else if (nums[i] > v) exch(nums, i, great--); else i++; } sortQuickThreeWay(nums, lo, less-1); sortQuickThreeWay(nums, great+1, hi); }public void sortColors(int[] nums) { sortQuickThreeWay(nums, 0, nums.length-1); } }

桶排序

class Solution {public void sortColors(int[] nums) { if (nums.length <= 1){ return; } int[] states = new int[3]; for (int i = 0; i < nums.length; i++){ states[nums[i]]++; } for (int j = 1; j < 3; j++){ states[j] += states[j-1]; } Arrays.fill(nums, 0, states[0], 0); Arrays.fill(nums, states[0], states[1], 1); Arrays.fill(nums, states[1], states[2], 2); } }