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.
 * <p>
 * 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;
            }
        }
    }
}