46. Permutations

Given a collection of distinct 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],
  [3,2,1]
]

思路：大绝招

public class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> list = new ArrayList<List<Integer>>();
        List<Integer> temp = new ArrayList<Integer>();
        traceback(list,temp,nums);
        return list;
    }

    public void traceback(List<List<Integer>> list, List<Integer> temp, int[] nums){
        if (temp.size()==nums.length){
            list.add(new ArrayList<Integer>(temp));
        }else{
            for (int i=0;i<nums.length;i++){
                if (temp.contains(nums[i])) continue;
                temp.add(nums[i]);
                traceback(list,temp,nums);
                temp.remove(temp.size()-1);
            }    
        }

    }
}

另一种写法：

public class Solution{
    public List<List<Integer>> permute(int[] nums){
        List<List<Integer>> res = new LinkedList<List<Integer>>();
        if(nums == null || nums.length == 0){
            return res;
        }

        permuteHelper(res, new LinkedList<Integer>(), nums, new HashSet<Integer>());
        return res;
    }

    public void permuteHelper(List<List<Integer>> res, List<Integer> clist, int[]nums, HashSet<Integer> set){
        if(clist.size()==nums.length){
            res.add(new LinkedList<Integer>(clist));
        }else{
            for(int i = 0; i< nums.length; i++){
                if(!set.contains(nums[i])){
                    clist.add(nums[i]);
                    int last = clist.size() - 1;
                    set.add(nums[i]);
                    permuteHelper(res,clist,nums,set);
                    set.remove(nums[i]);
                    clist.remove(last);//index, not value
                    }
                }
            }
    }
}