Permutations leetcode java
题目:
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]
.
题解:
这道题就用循环递归解决子问题。
因为求所有组合,这就意味着不能重复使用元素,要用visited数组。
有因为是所有可能的组合,所以循环length次,就是这里面每位都有可能有length个可能性。
正因为如此,每一层递归就不需要传递一个start点,告诉他从哪开始(因为都是从头开始循环)。
代码如下:
1 public ArrayList<ArrayList<Integer>> permute(int[] num) {
2 ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
3 ArrayList<Integer> item = new ArrayList<Integer>();
4
5 if(num.length==0||num==null)
6 return res;
7 boolean[] visited = new boolean[num.length];
8
9 permutation_helper(num,res,item,visited);
return res;
}
public void permutation_helper(int[] num, ArrayList<ArrayList<Integer>> res, ArrayList<Integer> item,boolean[] visited){
if(item.size()==num.length){
res.add(new ArrayList<Integer>(item));
return;
}
for(int i = 0; i<num.length;i++){
if(!visited[i]){
item.add(num[i]);
visited[i]=true;
permutation_helper(num,res,item,visited);
item.remove(item.size()-1);
visited[i]=false;
}
}
}
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