Permutations 排序（有重复数）II @LeetCode

比起http://blog.youkuaiyun.com/fightforyourdream/article/details/14217105

多加了一个while来去重，发现这个去重方法在另一道题也用过，同样也是DFS里面去重，很好用！

另外就是在最前面加了一个sort，因为如果没加，当输入乱序时就会Output Limit Exceed!

package Level4;

import java.util.ArrayList;
import java.util.Collections;

/**
 * Permutations II 
 * 
 * Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[1,1,2], [1,2,1], and [2,1,1].
 *
 */
public class S47 {

	public static void main(String[] args) {
		int[] num = {1,2,1};
		System.out.println(permuteUnique(num));
	}
	
	public static ArrayList<ArrayList<Integer>> permuteUnique(int[] num) {
		ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
		ArrayList<Integer> done = new ArrayList<Integer>();
		ArrayList<Integer> rest = new ArrayList<Integer>();
		for(int val : num){
			rest.add(val);
		}
		Collections.sort(rest);		// 这里要先排序一下。。否则遇到非递增排序的输入就会Output Limit Exceed!
		rec2(done, rest, ret);
		return ret;
	}
	
	public static void rec2(ArrayList<Integer> done, ArrayList<Integer> rest, ArrayList<ArrayList<Integer>> ret){
		if(rest.size() == 0){
			ret.add(new ArrayList<Integer>(done));
			return;
		}
		
		for(int i=0; i<rest.size(); i++){
			done.add(rest.get(i));
			ArrayList<Integer> newRest = new ArrayList<Integer>(rest);
			newRest.remove(i);
			rec2(done, newRest, ret);
			done.remove(done.size()-1);
			
			while(i<rest.size()-1 && rest.get(i)==rest.get(i+1)){
				i++;
			}
		}
	}
}

过滤重复元素方法同 http://blog.youkuaiyun.com/fightforyourdream/article/details/16859111

public class Solution {
    public ArrayList<ArrayList<Integer>> permuteUnique(int[] num) {
        ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
        ArrayList<Integer> done = new ArrayList<Integer>();
        ArrayList<Integer> rest = new ArrayList<Integer>();
        Arrays.sort(num);
        for(int val : num){
            rest.add(val);
        }
        rec(done, rest, ret);
        return ret;
    }
    
    public void rec(ArrayList<Integer> done, ArrayList<Integer> rest, ArrayList<ArrayList<Integer>> ret){
        if(rest.size()==0){
            ret.add(new ArrayList<Integer>(done));
            return;
        }
        
        for(int i=0; i<rest.size(); i++){
            int first = rest.remove(i);
            done.add(first);
            rec(done, rest, ret);
            done.remove(done.size()-1);
            rest.add(i, first);     // insert to its original position
            while(i+1<rest.size() && rest.get(i)==rest.get(i+1)){
                i++;
            }
        }
    }
}