Rotate Array

Rotate Array

Description

Given an array, rotate the array to the right by k steps, where k is non-negative.

Example 1:

Input: [1,2,3,4,5,6,7] and k = 3
Output: [5,6,7,1,2,3,4]
Explanation:
rotate 1 steps to the right: [7,1,2,3,4,5,6]
rotate 2 steps to the right: [6,7,1,2,3,4,5]
rotate 3 steps to the right: [5,6,7,1,2,3,4]

Example 2:

Input: [-1,-100,3,99] and k = 2
Output: [3,99,-1,-100]
Explanation: 
rotate 1 steps to the right: [99,-1,-100,3]
rotate 2 steps to the right: [3,99,-1,-100]

Tags: Array

Note：
- Try to come up as many solutions as you can, there are at least 3 different ways to solve this problem.
- Could you do it in-place with O(1) extra space?

解讀題意

給定一個數組，一個k，求數組向右前進k後（超過數組長度，末項變爲首項），數組變成了什麼？

思路1

這是一個數組反轉的問題。假設k=3，有一個[1,2,3,4,5,6,7]數組，那麼可以這麼求解：
1. 7 6 5 4 3 2 1（反轉全部數）
2. 5 6 7 4 3 2 1（反轉前k個數）
3. 5 6 7 1 2 3 4（反轉n-k個數）

class Solution {
     public void rotate(int[] nums, int k) {
        k %= nums.length;
        // 數組順序全部反過來
        reverse(nums, 0, nums.length - 1);
        // 反轉前k個
        reverse(nums, 0, k - 1);
        // 反轉n-k個
        reverse(nums, k, nums.length - 1);
    }

    public void reverse(int[] nums, int start, int end) {

        int temp = 0;
        while (start < end) {
            temp = nums[start];
            nums[start] = nums[end];
            nums[end] = temp;
            start++;
            end--;
        }
    }
}

leetCode彙總：https://blog.csdn.net/qingtian_1993/article/details/80588941

項目源碼，歡迎star：https://github.com/mcrwayfun/java-leet-code