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【一天一道LeetCode】#75. Sort Colors

來源：程序員人生發布時間：2016-06-15 14:34:38 閱讀次數：2683次

1天1道LeetCode

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（1）題目

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.

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 an one-pass algorithm using only constant space?

（2）解題

題目大意：數組包括0,1,2，依照大小排列這些數字。

1、最簡單的方法

利用STL的sort1句話就解決了。

class Solution {
public:
    void sortColors(vector<int>& nums) {
        sort(nums.begin(),nums.end());
    }
};

2、兩個指針

算法思想很簡單，把所有的0交換到隊首，把所有的1交換到隊尾

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int start = 0;
        int end = nums.size()-1;
        for(int i = 0 ; i<nums.size();i++)//把0交換到前面
        {
            if(nums[i]==0) {
                if(i!=start) swap(nums[i],nums[start]);
                start++;
            }
        }
        for(int i = nums.size()-1 ; i>=start ;i--)//把2交換到尾部
        {
            if(nums[i]==2){
                if(i!=end) swap(nums[i],nums[end]);
                end--;
            }
        }
    }
};

3、兩個指針優化版

保持3個指針i、j和k，從0~i表示1，i+1~j表示1，k~nums.size()表示2
代碼也比較簡單：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int i = 0, j = i, k = nums.size() - 1;
        while(j <= k){
            if(nums[j] == 0) swap(nums[i++], nums[j++]);//0交換到前面
            else if(nums[j] == 1) j++;//1保持不動
            else swap(nums[k--], nums[j]);//2交換到尾部
        }
    }
};