leetcode - Rotate Array

leetcode - 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?

Related problem: Reverse Words in a String II

class Solution {
public:
    void rotate(vector<int>& nums, int k) {
        vector<int> res;
        //vector<int>::iterator it1 = nums.begin();
        if(k==0) return;
        vector<int>::iterator it2 = nums.begin() + (nums.size() - k%nums.size());
        vector<int>::iterator it = it2;
        while(it!=nums.end()){
            res.push_back(*it);
            it++;
        }
        it = nums.begin();
        while(it!= it2){
            res.push_back(*it);
            it++;
        }
        nums = res;
    }
};

然而这个方法空间复杂度为n，还有一种空间复杂度为1的版本：

class Solution {
public:
    void rotate(int nums[], int n, int k) {
        k = k % n;
        reverse(nums, nums + n);
        reverse(nums, nums + k);
        reverse(nums + k, nums + n);
    }
};

 先把AB reverse一次得到reverse(B)reverse(A)

然后再把reverse(B),reverse(A)分别reverse一次就得到了BA

 

很巧妙。