[2018-Interview] Rotate Array

Original question: https://leetcode.com/problems/rotate-array/description/

Answer:
Solution 1): time O(n), space O(n)
Solution 2): time O(n), space O(1) - The underlying truth of rotation is actually reversing.
Solution 3): time O(n*k), space O(1)
Solution 4): time O(n), space O(1)- Basic logic of it is to move "smaller part" in the expected result first, then modify the remaining part, i.e. "bigger part" using same logic.

public class Solution {
    public void rotate(int[] nums, int k) {
        k = k % nums.length; 
        //(1) new array; 
        int[] oldNums = nums.clone();
        for(int i = 0; i< nums.length;i++){
            nums[(i+k)%nums.length] = oldNums[i]; 
        }
        
        //(2) reverse 3 times 
        // reverse(nums, 0, nums.length-1); 
        // reverse(nums, 0, k-1);
        // reverse(nums, k, nums.length-1); 
        
        //(3) move k times
        // while(k-->0){
        //     int tmp = nums[nums.length-1];
        //     for(int i = nums.length-1; i>0; i--){
        //         nums[i] = nums[i-1];
        //     }
        //     nums[0] = tmp; 
        // }
    }
    
    public void reverse(int[] nums, int start, int end){
        while(start < end){
            int temp = nums[start]; 
            nums[start] = nums[end]; 
            nums[end]=temp; 
            start++; end--; 
        }
    }
}

Solution 4)

public class Solution {
    public void rotate(int[] nums, int k) {
        if (nums == null || k <= 0) {
            return;
        }
        helper(nums, 0, nums.length - 1, k % nums.length);
    }
    
    private void helper(int[] nums, int left, int right, int k) {
        if ( k <= 0 || left >= right) {
            return;
        }
        int steps = right - left + 1 - k;
        int temp;
        if (steps > k) {
            for (int i = 0; i < k; i ++) {
                temp = nums[i + left];
                nums[i + left] = nums[left + steps + i];
                nums[left + steps + i] = temp;
            }
            helper(nums, left + k, right, k);
        } else {
            if (steps < k) {
                for (int i = 0; i < steps; i ++) {
                    temp = nums[right - i];
                    nums[right - i] = nums[right - k - i];
                    nums[right - k - i] = temp;
                }
                helper(nums, left, right - steps, k - steps);
            } else {
                for (int i = 0; i < steps ; i ++) {
                 temp = nums[i + left];
                nums[i + left] = nums[left + k + i];
                nums[left + k + i] = temp;                   
                }
                return;
            } 
        }
        
    }
}