LeetCode 189. Rotate Array (旋转数组)

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


题目标签:Array

  题目给了我们一个数组 和 k。 让我们 旋转数组 k 次。

  这里有一个很巧妙的方法:

    利用数组的length - k 把数组 分为两半;

    reverse 左边和右边的数组;

    reverse 总数组。

  举一个例子:

  1 2 3 4 5 6 7  如果k = 3 的话, 会变成 5 6 7 1 2 3 4

  1 2 3 4 5 6 7  middle = 7 - 3 = 4,分为左边 4个数字,右边 3个数字

  4 3 2 1 7 6 5  分别把左右reverse 一下

  5 6 7 1 2 3 4  把总数组reverse 一下就会得到答案

Java Solution:

Runtime beats 15.37%

完成日期:04/13/2017

关键词:Array

关键点:利用k把数组分两半;reverse左右两边数组;reverse总数组

 public class Solution
{
public void rotate(int[] nums, int k)
{
if(nums == null || nums.length == 0 || k % nums.length == 0)
return; int turns = k % nums.length;
int middle = nums.length - turns; reverse(nums, 0, middle-1); // reverse left part
reverse(nums, middle, nums.length-1); // reverse right part
reverse(nums, 0, nums.length-1); // reverse whole part
} public void reverse(int[] arr, int s, int e)
{
while(s < e)
{
int temp = arr[s];
arr[s] = arr[e];
arr[e] = temp; s++;
e--;
}
}
}

参考资料:

http://www.cnblogs.com/grandyang/p/4298711.html

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