Rotate Array

189. Rotate Array

requirement: 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 problme.

strategy 1:

using arraycopy(Object src, int srcPos, Object dest, int destPos, int length) from the Java APIlibrary

1. first create a new array called newList, let k= length, n = src.length

2. copy the subarray in src from srcPos(n-k) to srcPos(n-k) + k-1 substring into the new array at the position from dest(0) to dest(0) + k -1 

3. then copy the subarray in src from srcPos(0) to srcPos(k)+n-k-1 substring into the new array position from dest(k) to dest(k) + n-k -1

4. then copy the whole new string into the original string 

public class Solution{

   public void rotate(int[] nums, int k){

       int n = nums.length;

       int[] newList = new int
;

       if (n > 1){

          k = k % n;

          System.arraycopy(nums, n-k, newList, 0, k);

          System.arraycopy(nums, 0, newList, k, n-k);

          System.arraycopy(newList, 0, nums, 0, n);

       }

   }

}

strategy 2:

1. first reverse the substring (0, n-k-1)

2. then reverse the substring (n-k, n-1)

3. finally reverse the whole string and return the new string

total space O(1)

public class Solution{

  public void rotate(int[] nums, int k){

      if(nums == null || nums.length < 2){

         return;

      }

      k = k % nums.length;

      reverse(nums, 0, nums.length-k-1);

      reverse(nums, nums.length-k, nums.length-1);

      reverse(nums, 0, nums.length-1);

   }

   private void reverse(int[] nums, int i, int j){

       int tmp = 0;

       while(i<j){

          tmp = nums[i];

          nums[i] = nums[j];

          nums[j] = tmp;

          i++;

          j--;

       }

    }

}

strategy 3

1. using stack to store the substring (n-k, n-1)

2. then replace array nums[i+k] = nums[i] from n-k-1 to 0

3. then assign substring (0, k-1) with the stack element using pop() function

public class Solution{

  public void rotate(int[] nums, int k){

      if(nums == null || nums.length < 2){

         return;

      }

      k = k % nums.length;

      Stack<Integer> stack = new Stack<Integer>();

      for(int i= nums.length-1; i>= nums.length - k; i--){

          stack.push(nums[i]);

      }

      for(int i= nums.length-k-1; i>=0; i--){

            nums[i+k]=nums[i];

      }

      for(int i=0; i<k; i++){

         nums[i]=stack.pop();

      }

   }

}