46. Permutations

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LeetCode

Approach 1: Backtracking

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class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> output = new LinkedList();

        ArrayList<Integer> nums_lst = new ArrayList<Integer>();
        for (int num : nums) nums_lst.add(num);

        int n = nums.length;
        backtrack(n, nums_lst, output, 0);
        return output;
      }
    
    public void backtrack(int n,ArrayList<Integer> nums,List<List<Integer>> output,int first) {
        if (first == n) output.add(new ArrayList<Integer>(nums));
        for (int i = first; i < n; i++) {
          Collections.swap(nums, first, i);
          backtrack(n, nums, output, first + 1);
          Collections.swap(nums, first, i);
        }
      }
}

Approach 2: using permutation generation -> Heap Algorithm

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class Solution {
    private void swap(int[] a, int i, int j) {
        int tmp = a[i];
        a[i] = a[j];
        a[j] = tmp;
    }
    // Generating permutation using Heap Algorithm
    private void heapPermutation(int[] arr, int n,List<List<Integer>> res) {
        // if n becomes 1 then prints the obtained permutation
        if (n == 1) {
            ArrayList<Integer> ls=new ArrayList<>();
            for(int i:arr) ls.add(i);
            res.add(ls);
            return;
        }

        for (int i = 0; i < n; i++) {
            heapPermutation(arr, n - 1,res);

            if (n % 2 == 1) { // if n is odd, swap first and last element
                swap(arr, 0, n - 1);
            } else { // If size is even, swap i-th and last element
                swap(arr, i, n - 1);
            }
        }
    }
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res=new ArrayList<>();
        heapPermutation(nums,nums.length,res);
        return res;
    }
}

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