189-RotateArray.go

package main

// 189. Rotate Array
// Given an integer array nums, rotate the array to the right by k steps, where k is non-negative.

// Example 1:
// Input: nums = [1,2,3,4,5,6,7], 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: nums = [-1,-100,3,99], 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]

// Constraints:
//     1 <= nums.length <= 10^5
//     -2^31 <= nums[i] <= 2^31 - 1
//     0 <= k <= 10^5
 
// Follow up:
//     Try to come up with as many solutions as you can. There are at least three different ways to solve this problem.
//     Could you do it in-place with O(1) extra space?

import "fmt"

//  时间复杂度 O(n)，空间复杂度 O(1)
// 末尾 k mod n 个元素移动至了数组头部，剩下的元素右移 k mod n 个位置至最尾部。确定了最终态以后再变换就很容易。
// 先将数组中所有元素从头到尾翻转一次，尾部的所有元素都到了头部，然后再将 [0,(k mod n) − 1] 区间内的元素翻转一次，
// 最后再将 [k mod n, n − 1] 区间内的元素翻转一次，
func rotate(nums []int, k int) {
    k %= len(nums)
    reverse := func(a []int) {
        for i, n := 0, len(a); i < n/2; i++ {
            a[i], a[ n-1-i ] = a[ n-1-i ], a[i]
        }
    }
    reverse(nums) // 先将数组中所有元素从头到尾翻转一次，尾部的所有元素都到了头部，
    //fmt.Printf("reverse(nums) = %v\n",nums)
    reverse(nums[:k]) // 再将 [0,(k mod n) − 1] 区间内的元素翻转一次
    //fmt.Printf("reverse(nums[:k]) = %v\n",nums)
    reverse(nums[k:]) // 再将 [k mod n, n − 1] 区间内的元素翻转一次
    //fmt.Printf("reverse(nums[k:]) = %v\n",nums)
}

//  时间复杂度 O(n)，空间复杂度 O(n)
//  使用一个额外的数组，先将原数组下标为 i 的元素移动到 (i+k) mod n 的位置，再将剩下的元素拷贝回来即可。
func rotate1(nums []int, k int) {
    newNums := make([]int, len(nums))
    // 先将原数组下标为 i 的元素移动到 (i+k) mod n 的位置
    for i, v := range nums {
        //fmt.Printf("(i+k) %% len(nums) = %v\n", (i+k) % len(nums))
        newNums[(i+k) % len(nums)] = v
    }
    copy(nums, newNums) // 再将剩下的元素拷贝回来
}

func main() {
    // 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]
    nums1 := []int{ 1,2,3,4,5,6,7 }
    fmt.Printf("before: nums1 = %v\n", nums1) // nums = [1,2,3,4,5,6,7], k = 3
    rotate(nums1, 3)
    fmt.Printf("after: nums1 = %v\n", nums1) // [5,6,7,1,2,3,4]

    // rotate 1 steps to the right: [99,-1,-100,3]
    // rotate 2 steps to the right: [3,99,-1,-100]
    nums2 := []int{ -1,-100,3,99 }
    fmt.Printf("before: nums2 = %v\n", nums2) // [-1,-100,3,99]
    rotate(nums2, 2)
    fmt.Printf("after: nums2 = %v\n", nums2) // [3,99,-1,-100]


    nums11 := []int{ 1,2,3,4,5,6,7 }
    fmt.Printf("before: nums11 = %v\n", nums11) // nums = [1,2,3,4,5,6,7], k = 3
    rotate1(nums11, 3)
    fmt.Printf("after: nums11 = %v\n", nums11) // [5,6,7,1,2,3,4]

    nums12 := []int{ -1,-100,3,99 }
    fmt.Printf("before: nums12 = %v\n", nums12) // [-1,-100,3,99]
    rotate1(nums12, 2)
    fmt.Printf("after: nums12 = %v\n", nums12) // [3,99,-1,-100]
}