Permutations.java

package backtracking;

import java.util.ArrayList;
import java.util.List;

/**
 * Created by gouthamvidyapradhan on 15/03/2017.
 Given a collection of distinct numbers, return all possible permutations.

 For example,
 [1,2,3] have the following permutations:
 [
 [1,2,3],
 [1,3,2],
 [2,1,3],
 [2,3,1],
 [3,1,2],
 [3,2,1]
 ]
 */
public class Permutations
{
    /**
     * Main method
     * @param args
     * @throws Exception
     */
    public static void main(String[] args) throws Exception
    {
        int[] nums = {1, 2, 3};
        List<List<Integer>> result = new Permutations().permute(nums);
    }

    public List<List<Integer>> permute(int[] nums)
    {
        List<List<Integer>> result = new ArrayList<>();
        nextPermutation(0, nums, result);
        return result;
    }

    private void nextPermutation(int i, int[] nums, List<List<Integer>> result)
    {
        if(i == nums.length - 1)
        {
            List<Integer> list = new ArrayList<>();
            for(int n : nums)
                list.add(n);
            result.add(list);
        }
        else
        {
            for(int j = i, l = nums.length; j < l; j++)
            {
                int temp = nums[j];
                nums[j] = nums[i];
                nums[i] = temp;
                nextPermutation(i + 1, nums, result);
                temp = nums[j];
                nums[j] = nums[i];
                nums[i] = temp;
            }
        }
    }
}