Leetcode-189 Rotate Array

#189.    Rotate Array          

Rotate an array of n elements to the right by k steps.

For example, with n = 7 and k = 3, the array [1,2,3,4,5,6,7] is rotated to [5,6,7,1,2,3,4].

Note:
Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.

[show hint]

Hint:
Could you do it in-place with O(1) extra space?

题解：最容易想到的是右移k次，用最容易想到的方法，结果是超时，时间复杂度是O(kn).

class Solution {
public:
    void rotate(vector<int>& nums, int k) {
       
        int n=nums.size();
        
        while(k--)
        {
           rightShift(nums);
        }
    }
    void rightShift(vector<int>& nums)
    {
        int temp=0;
        temp=nums[nums.size()-1];
        for(int i=nums.size()-1;i>0;i--)
        {
            nums[i]=nums[i-1];
        }
        nums[0]=temp;
    }
};

只好换三步反转法来做，时间复杂度是O(n),空间复杂度是O(1)

class Solution {
public:
    void rotate(vector<int>& nums, int k) {
        int n=nums.size();
        k=k%n;
        if(k==0)
            return ;
            
        reverseString(nums,0,n-k-1);
        reverseString(nums,n-k,n-1);
        reverseString(nums,0,n-1);
    }
    void reverseString(vector<int>& nums,int from,int to)
    {
        while(from<to)
        {
            int temp=nums[from];
            nums[from++]=nums[to];
            nums[to--]=temp;
        }
    }
};