题目

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]

Related problem: Reverse Words in a String II

Credits:
Special thanks to @Freezen for adding this problem and creating all test cases.

大意

旋转数组

答案

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class Solution {
public:
    void rotate(vector<int>& nums, int k) {
        int len =nums.size();
        k = k%len;
        reverse(nums,0,len-1);
        reverse(nums,0,k-1);
        reverse(nums,k,len-1);
        return;
    }
    void reverse(vector<int>& nums,int left, int right)
    {
        int len = nums.size();
        while(left<right)
        {
            swap(nums[left],nums[right]);
            left++;
            right--;
        }
    }
    
};

思路

很简单的一道题，但是毕竟第一次见。

三步反转法来做，时间复杂度是O(n),空间复杂度是O(1)

一共有3步。假设输入数组的下标是0~ n-1，需要rotate的步数是k.

step 1 reverse原来的数组

step 2 reverse 0~ k-1

step 3 reverse k ~ n-1