Permutations solve

week 3/Solution.java

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+import java.util.ArrayList;
+import java.util.List;
+
+class Solution {
+
+  /**
+   * Main Method to generate all possible permutations of the given array of integers
+   * @param nums The array of integers to permute.
+   * @return a List of lists, where each inner list is a posible permutation the array
+   */
+  public List<List<Integer>> permute(int[] nums) {
+    // List to store all the permutations
+    List<List<Integer>> result = new ArrayList<>();
+
+    // Start the recursive generation of permutations
+    generatePermutations(nums, nums.length, result);
+
+    // Return the list of all permutations
+    return result;
+  }
+
+  /**
+   * Recursive method that generates all permutations of the array using Heap's algorithm
+   * @param nums The array of integers to permute.
+   * @param size The current size of the subset to permute.
+   * @param result The list where all permutations are stored.
+   */
+  private void generatePermutations(int[] nums, int size, List<List<Integer>> result) {
+    // Base case: If the size of the subset is 1, add the current permutation to the result
+    if (size == 1) {
+      // Create a new list to store the current permutation
+      List<Integer> permutation = new ArrayList<>();
+
+      // Add each elemnt from the array
+      for (int num : nums) {
+        permutation.add(num);
+      }
+
+      // Add the current permutation to the result list.
+      result.add(permutation);
+    } else {
+      // Recursively generate permutations for size - 1
+      for (int i = 0; i < size; i++) {
+        generatePermutations(nums, size - 1, result);
+
+        // Determine whether to swap the first or i-th element with the last element
+        // Based on the whether the current size is even or odd
+        if (size % 2 == 0) {
+          // If the size is even, swap the i-th element with the last element
+          swap(nums, i, size - 1);
+        } else {
+          // If the size is odd, swap the first element with the last element
+          swap(nums, 0, size - 1);
+        }
+      }
+    }
+  }
+
+  /**
+   * Helper method to swap two elements in the array.
+   * @param nums The array in which to swap elements.
+   * @param i The index of the first element to swap.
+   * @param j The index of the second element to swap
+   */
+  private void swap(int[] nums, int i, int j) {
+    int temp = nums[i];
+    nums[i] = nums[j];
+    nums[j] = temp;
+  }
+
+  public static void main(String[] args) {
+    Solution pg = new Solution();
+    int[] nums = { 1, 2, 3, 4 };
+    List<List<Integer>> permutations = pg.permute(nums);
+    for (List<Integer> permutation : permutations) {
+      System.out.println(permutation);
+    }
+  }
+}