全排列算法

做了第三题-搭积木和 第六题-寒假作业，都用了全排列。

用dfs写全排列：

/*比如从1-9,思路：让a[1]-a[9]每个数从1-9遍历，用visit[10]
 *记录，如果用过了，置为false.*/

package test;


public class Main {
	static int count = 0;
	public static void main(String[] args) {
		int a[] = new int[10];
		boolean visit[] = new boolean[10];
		dfs(a,visit,1);
		System.out.println(count);
	}
	private static void dfs(int[] a, boolean[] visit, int num) {
		if (num==10) {
			count++;
			return;
		}
		
		for (a[num] = 1; a[num] <= 9; a[num]++) {
			if (judge(a)) {//这里根据情况剪枝，减少计算量
				if (visit[a[num]]==false) {
					visit[a[num]]=true;
					num = num + 1;
					dfs(a, visit, num);
					num = num -1;
					visit[a[num]]=false;
				}
			}else {
				continue;
			}
		}
	}
	private static boolean judge(int[] a) {
		
		return true;
	}
}

在网上又查到了效率高一点的全排列算法

package test;

public class Main {

	static int count = 0;
	
	/**
	 * @param args
	 */
	public static void main(String[] args) {
		int a[] = new int[10];
		for (int i = 0; i < a.length; i++) {
			a[i] = i;
		}
		permulation(a, 0, a.length);
		System.out.println(count);
		
	}
	
	public static void permulation(int[] list, int start, int length ) {
        int i;
        if (start == length) {
        	if (juedje(list)) {//判断
        		count +=1;
			}
            
        } else {
            for (i = start; i < length; i++) {
                swap(list, start, i);
                permulation(list, start + 1, length);
                swap(list, start, i);
            }
        }
    }


	private static boolean juedje(int[] list) {
		return true;
	}

	public static void swap(int[] list, int start, int i) {
        int temp;
        temp = list[start];
        list[start] = list[i];
        list[i] = temp;
    }
}