Permutations

Permutations

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].

Java代码：

public static List<List<Integer>> permute(int[] num) {
		LinkedList ans = new LinkedList<>(); // queue
		ans.offer(new LinkedList<Integer>()); // init with empty list
		// loop through sizes of lists used for inserting new element in asc
		// order
		for (int n = 0; n < num.length; n++) {
			// loop through all lists of a current size
			while (n == ((List<List<Integer>>) ans.peek()).size()) {
				List<Integer> l = (List<Integer>) ans.poll(); // next "base"
																// list of a
																// curr size
				// loop through all position in a "base" list generating new
				// list
				for (int i = 0; i <= l.size(); i++) {
					LinkedList<Integer> newL = new LinkedList<>(l.subList(0, i));
					newL.add(num[n]);
					newL.addAll(l.subList(i, l.size()));
					ans.offer(newL);
				}
			}
		}
		return ans;
	}

	public static void main(String[] arqs) {
		int[] num = new int[] { 1, 2, 3 };
		List<List<Integer>> result = permute(num);
		Iterator ite = result.iterator();
		while (ite.hasNext()) {
			List<Integer> list = (List<Integer>) ite.next();
			Iterator ite_2 = list.iterator();
			System.out.print("{");
			while (ite_2.hasNext()) {
				int x = (Integer) ite_2.next();
				System.out.print(x);
			}
			System.out.print("}\n");
		}
	}