notes/OJ notes/pages/Leetcode Rotate-Array.md

Leetcode Rotate-Array

2022-07-10 08:54

Algorithms:

#algorithm #array_in_place_operation #reverse_array

Data structures:

#DS #array

Difficulty:

#coding_problem #difficulty-medium

Additional tags:

#leetcode #CS_list_need_practicing #CS_list_need_understanding

Revisions:

N/A

Related topics:

Links:

• Link to problem

---

Problem

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

Examples

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 <= 105
• -231 <= nums[i] <= 231 - 1
• 0 <= k <= 105

Thoughts

Method 1: Array In-place operation

This one is hard. ==#TODO: Revisit this one some other day==

• Reversing a subarray doesn't change the continuity, the neighbors inside the subarray is same.
• Reversing a subarray changes the edge's neighbors

Suppose a array looks like:

[1, 2,| 3, 4, 5]

If reverse all two subarrays, the neighbors isn't changed only the orders are changed

[1, 2,| 3, 4, 5]:    5-1-2, 1-2-3, 2-3-4, 4-5-1
[2, 1,| 5, 4, 3]:    3-2-1, 2-1-5, 5-1-4, 4-3-2

When reversed back, the order is changed back to original, but the two sub-arrays are swapped

[3, 4, 5,| 1, 2]

By doing this, the order in inner-array isn't changed, but the order of all is changed.

Method 2: Copy

Watch out for integer overflow.

Method 3: CPP's STL sway subarrays

Remember to sanitize k, when k > size

Solution

Method 1:

class Solution {
  void swap(vector<int> &nums, int l, int r) {
    int temp = nums[l];
    nums[l] = nums[r];
    nums[r] = temp;
  }
  void reverse(vector<int> &nums, int l, int r) {
    while (l < r) {
      swap(nums, l, r);
      l++;
      r--;
    }
  }

public:
  void rotate(vector<int> &nums, int k) {
    const int size = nums.size();
    k = k % size;
    // Reversion
    reverse(nums, 0, size - k - 1);
    reverse(nums, size - k, size - 1);
    reverse(nums, 0, size - 1);
  }
};

Method 2:

class Solution {
  void swap(vector<int> &nums, int l, int r) {
    int temp = nums[l];
    nums[l] = nums[r];
    nums[r] = temp;
  }

public:
  void rotate(vector<int> &nums, int k) {
    int size = nums.size();
    if (size <= 1) {
      return;
    }
    // Copy
    k = size - k;
    while (k < 0) {
      k += size;
    }
    vector<int> ans(nums.size());
    for (int i = 0; i < size; i++) {
      ans[i] = nums[(i + k) % size];
    }
    for (int i = 0; i < size; i++) {
      nums[i] = ans[i];
    }
  }
};

Method 3:

class Solution {
  void swap(vector<int> &nums, int l, int r) {
    int temp = nums[l];
    nums[l] = nums[r];
    nums[r] = temp;
  }

public:
  void rotate(vector<int> &nums, int k) {
    int size = nums.size();
    if (size <= 1) {
      return;
    }
    // STL
    k = k % size;
    k = size - k;
    while (k < 0) {
      k += size;
    }

    nums.insert(nums.end(), nums.begin(), nums.begin() + k);
    nums.erase(nums.begin(), nums.begin() + k);
  }
};