LeetCode::Rotate Array C语言

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]

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

Related problem: Reverse Words in a String II

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

分析

最简单的方法就是每次整体向右移动一个位置，但是实在是太慢了。

观察可发现其实每个元素的最终目的位置是可以算出来的（因为唯一确定），公式（index_new = (index + k)%n）所以理论上每个元素只需要移动一次。

题目要求只用 O(1)的extra space，也就不能重开一个数组了，虽然那样子非常直观而且快。

我的实现是从第一个元素开始移动，基本是三步，见代码注释。

注意到每个元素必须移动也只移动一次，所以当移动次数达到n的时候，循环要终止。

C语言代码

#include <stdio.h>

#include <QDebug>

void printNums(int nums[], int n)

{

    for (int index = 0; index < n; ++index)

    {

        printf("%d ", nums[index]);

    }

    printf("\n");

}

void rotate(int nums[], int n, int k)

{

    //all elements should be adjusted once and only once

    //so we should stop if count == n

    int count = 0;

    for (int i = 0; i <n ; ++i)

    {

        int index = i;

        int tmp = nums[index];

        while(1)

        {

            count++;

            // we are dealing with index

            // it will be moved to index_new

            int index_new = (index + k) % n;

            // let store nums[index_new] since it will be covered

            nums[i] = nums[index_new];

            // move completed

            nums[index_new] = tmp;

            // information for the next element

            tmp = nums[i];

            index = index_new;

            printNums(nums,n);

            // if we have done a circle, break

            if (i == index)

            {

                break;

            }

        }

        if (count == n)

        {

            break;

        }

    }

}

int main()

{

    int nums[] = {1,2,3,4,5,6,7,8,9};

    int n = 9;

    int k = 8;

    rotate(nums, n, k);

    printNums(nums, n);

}