46.permute

46.全排列

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> res=new ArrayList<>();
        List<Integer> output=new ArrayList();
        for(int num:nums){
            output.add(num);
        }
        int n=nums.length;
        backtrack(res,output,n,0);
        return res;

    }
    public void backtrack(List<List<Integer>> res,List<Integer> output,int n,int first){
        if(first==n){
            res.add(new ArrayList(output));
            return;
        }
        for(int i=first;i<n;i++){
            Collections.swap(output,i,first);
            backtrack(res, output, n, first+1);
            Collections.swap(output,i,first);
        }
    }
}