Move Zeroes

import java.util.Arrays;

class Main {
    public static void main(String[] args) {
        /**
         * Problem: Move Zeroes
         * Given an array, move all 0's to the end while maintaining the relative order of non-zero elements.
         * 
         * Example:
         * 
         * Input: [0, 1, 0, 3, 12]
         * Output: [1, 3, 12, 0, 0]
         */
         
         int[] input =  {0, 1, 0, 3, 12};
         
         // We can use two nested loops to iterate each element and comparing it with the
         // next element to decide if we can move it to the next position because it is 0 or not.
         // However, this is not the most efficient approach.
         
         // Instead, we can use Two pointers approach with same direction.
         // Basicaslly, we can set a slow index in the first element to start the comparison
         // The fast element will traverse the rest of the elements of the array.
         // We can switch the elements if the element at soft position is 0 and 
         // the element at fast position is not 0
         // In this case, we can move slow position one step (slow++)
         
         System.out.println(Arrays.toString(moveZeroes(input)));
         
         /**
          * 🧪 Test Your Code with These Cases:
          * [0, 1, 0, 3, 12] → [1, 3, 12, 0, 0]
          * [1, 0, 0, 3, 12] → [1, 3, 12, 0, 0]
          * [0, 0, 1] → [1, 0, 0]
          * [1, 2, 3] → [1, 2, 3] (no zeros)
          * [0, 0, 0] → [0, 0, 0] (all zeros)
          */
          input = new int[]{1, 0, 0, 3, 12};
          System.out.println(Arrays.toString(moveZeroes(input)));
          
          input = new int[]{0, 0, 1};
          System.out.println(Arrays.toString(moveZeroes(input)));
          
          input = new int[]{1, 2, 3};
          System.out.println(Arrays.toString(moveZeroes(input)));
          
          input = new int[]{0, 0, 0};
          System.out.println(Arrays.toString(moveZeroes(input)));
          
          input = new int[]{};
          System.out.println(Arrays.toString(moveZeroes(input)));
          
          input = null;
          System.out.println(Arrays.toString(moveZeroes(input)));
    }
    
    private static int[] moveZeroes(int[] input) {
        if (input == null || input.length == 0) {
            return input;
        }
        
        int slow = 0; // Space O(1)
         for (int fast = 0; fast < input.length; fast++) { // Time O(n)
             if (input[fast] != 0) {
                 // switch elements
                 int temp = input[fast];
                 input[fast] = input[slow];
                 input[slow] = temp;
                 
                 slow++;
             }
         }
         
         // Time complexity O(n)
         // Space complexity U(1)
         
         return input;
    }
}