CODE 87. Permutations

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Given a collection of 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],
and [3,2,1].

public ArrayList<ArrayList<Integer>> permute(int[] num) {
	// Note: The Solution object is instantiated only once and is reused by
	// each test case.
	ArrayList<Integer> numbers = new ArrayList<Integer>();
	for (int n : num) {
		numbers.add(n);
	}
	return dfs(numbers);
}

ArrayList<ArrayList<Integer>> dfs(ArrayList<Integer> numbers) {
	if (numbers.size() <= 1) {
		ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
		ArrayList<Integer> result = new ArrayList<Integer>(numbers);
		results.add(result);
		return results;
	}
	ArrayList<ArrayList<Integer>> results = new ArrayList<ArrayList<Integer>>();
	ArrayList<Integer> newNumbers = new ArrayList<Integer>(numbers);
	for (int i = 0; i < numbers.size(); i++) {
		Integer number = numbers.get(i);
		newNumbers.remove(number);
		ArrayList<ArrayList<Integer>> res = dfs(newNumbers);
		for (ArrayList<Integer> re : res) {
			re.add(0, number);
			results.add(re);
		}
		newNumbers.add(number);
	}
	return results;
}