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.

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

Related problem: Reverse Words in a String II

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解题思路：

要想满足O(1) space and in O(n) time, 只有一种方法： reversal. 

Assuming we are given {1,2,3,4,5,6} and order 2. The basic idea is:

1. Divide the array two parts: 1,2,3,4 and 5, 6

2. Rotate first part: 4,3,2,1,5,6

3. Rotate second part: 4,3,2,1,6,5

4. Rotate the whole array: 5,6,1,2,3,4

或者

(1) reverse the array;

(2) reverse the first k elements;

(3) reverse the last n-k elements.

这个方法与 Leetcode Reverse Words in a String II 达成一致。

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Java code:

public class Solution {
    public void rotate(int[] nums, int k) {
       int len = nums.length;
       k %= len;
       reverse(nums, 0, len-1);
       reverse(nums, 0, k-1);
       reverse(nums, k, len-1);
    }
    
    public void reverse(int[] nums, int l, int r){
        while(l < r){
            int temp = nums[l];
            nums[l] = nums[r];
            nums[r] = temp;
            l++;
            r--;
        }
    }
}

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Reference:

1. http://www.cnblogs.com/EdwardLiu/p/4306556.html