[算法]Rotate Array

You may have been using Java for a while. Do you think a simple Java array question can be a challenge? Let's use the following problem to test.

Problem: 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]. How many different ways do you know to solve this problem?

Solution 1 - Intermediate Array

In a straightforward way, we can create a new array and then copy elements to the new array. Then change the original array by using System.arraycopy().

public void rotate(int[] nums, int k) {
    if(k > nums.length) 
        k=k%nums.length;
 
    int[] result = new int[nums.length];
 
    for(int i=0; i < k; i++){
        result[i] = nums[nums.length-k+i];
    }
 
    int j=0;
    for(int i=k; i<nums.length; i++){
        result[i] = nums[j];
        j++;
    }
 
    System.arraycopy( result, 0, nums, 0, nums.length );
}

Space is O(n) and time is O(n). You can check out the difference between System.arraycopy() and Arrays.copyOf().

Solution 2 - Bubble Rotate

Can we do this in O(1) space?

This solution is like a bubble sort.

public static void rotate(int[] arr, int order) {
	if (arr == null || order < 0) {
	    throw new IllegalArgumentException("Illegal argument!");
	}
 
	for (int i = 0; i < order; i++) {
		for (int j = arr.length - 1; j > 0; j--) {
			int temp = arr[j];
			arr[j] = arr[j - 1];
			arr[j - 1] = temp;
		}
	}
}

However, the time is O(n*k).

Solution 3 – Reversal

Can we do this in O(1) space and in O(n) time? The following solution does.

Assuming we are given {1,2,3,4,5,6} and order 2. The basic idea is:

1. Divide the array two parts: 1,2,3,4 and 5, 6
2. Rotate first part: 4,3,2,1,5,6
3. Rotate second part: 4,3,2,1,6,5
4. Rotate the whole array: 5,6,1,2,3,4

public static void rotate(int[] arr, int order) {
	order = order % arr.length;
 
	if (arr == null || order < 0) {
		throw new IllegalArgumentException("Illegal argument!");
	}
 
	//length of first part
	int a = arr.length - order; 
 
	reverse(arr, 0, a-1);
	reverse(arr, a, arr.length-1);
	reverse(arr, 0, arr.length-1);
 
}
 
public static void reverse(int[] arr, int left, int right){
	if(arr == null || arr.length == 1) 
		return;
 
	while(left < right){
		int temp = arr[left];
		arr[left] = arr[right];
		arr[right] = temp;
		left++;
		right--;
	}	
}