Sword finger offer 38 Arrangement of strings

First, fix the first position n Kind of , Then the second one has (n - 1) Kind of , By analogy .
When there are duplicate characters in the string , There will also be repeated schemes in the arrangement . To eliminate duplicate schemes , When fixing a character , Guarantee “ Each character is fixed only once in this bit ” , That is, when repeated characters are encountered, they are not exchanged , Just skip . from DFS Perspective , This operation is called “ prune ” .

class Solution {
    
    List<String> res = new ArrayList<>();
    char[] arr;
    public String[] permutation(String s) {
    
        arr = s.toCharArray();
        dfs(0);
        return res.toArray(new String[res.size()]);
    }
    // Backtracking function 
    void dfs(int x){
    
        if(x == arr.length-1){
    
            res.add(String.valueOf(arr));
            return;
        }
        Set<Character>set = new HashSet<>();
        for(int i = x; i<arr.length; i++){
    
            if(set.contains(arr[i])){
    continue;}// Repeated pruning 
            set.add(arr[i]);
            swap(i,x);
            dfs(x+1);
            swap(i,x);
        }
    }
    // Exchange function 
    void swap(int a,int b){
    
        char ch = arr[a];
        arr[a] = arr[b];
        arr[b] = ch;
    }
}

The recursive aspect of intermediate exchange is not very clear .