子集的全排列

比如 123
1
2
3
12
21
13
31
23
32
123
132
213
231
312
321

public class SubPerm {
	public static void main(String[] args) {

		int[] a = { 1, 2, 3, 4 };
		System.out.println(perm(a, 0));
	}

	public static int perm(int[] a, int begin) {
		int count=0;		
		for (int i = 0; i < begin; i++) {
			System.out.print(a[i] + " ");			
		}
		if(begin>0){
			count=1;
			System.out.println();
		}
		if (begin == a.length) {
			return 1;
		}

		for (int i = begin; i < a.length; i++) {
			swap(a, begin, i);
			count += perm(a, begin + 1);
			swap(a, begin, i);
		}
		return count;
	}

	public static void swap(int[] a, int begin, int end) {
		int temp = a[begin];
		a[begin] = a[end];
		a[end] = temp;
	}
}