为什么 +1 在这里有效，而 ++ 运算符却没有

Question

I am preparing for interviews and trying to practice recursion , here is my solution to a problem that ask to generate 'n' parenthesis ie if n=1 then[[()]] if n=2 then [[()()], [(())]] and so on..

Here is my solution which works but if I increment the open before calling the recursive function it doesn't work, I have list both in the below code, not sure why this is happening :

 public List<String> generateParenthesis(int n) {
        List<String> rightAns = new ArrayList<>();
        List<String> wrongAns = new ArrayList<>();
        
        helper(0, 0, "", rightAns, n);     //["((()))","(()())","(())()","()(())","()()()"] for n = 3
        helperWrong(0, 0, "", wrongAns, n);//["((()))"] for n = 3
        
        return rightAns;
    }
    
    public void helper(int open, int close, String cur, List<String> ans, int n){
        if(cur.length() == 2*n){
            ans.add(cur);
            return;
        }
        
        if(open < n){
            //open++; doesn't work
            helper(open+1, close, cur+"(", ans, n);
            
        }
        
        if(close < open){
            //close++; doesn't work
            helper(open, close+1, cur+")", ans, n);
        }
        System.out.println("cur"+cur.toString());
        return;
            
    }
    
    public void helperWrong(int open, int close, String cur, List<String> ans, int n){
        if(cur.length() == 2*n){
            ans.add(cur);
            return;
        }
        
        if(open < n){
            open++; 
            helperWrong(open, close, cur+"(", ans, n);
            
        }
        
        if(close < open){
            close++; 
            helperWrong(open, close, cur+")", ans, n);
        }
        System.out.println("cur"+cur.toString());
        return;
            
    }

Answer 1

The postfix ++ operator is evaluated after the expresion. For example:

a=2;
b=a++; //b=2, a=3

Same when calling functions:

a=2;
foo(a++);
//a=3

void foo(int v){
  //v=2
}

So your recursive function will receive the value before it is incremented.

Answer 2

尝试++open和++close ，并且 cdalxndr 已经解释了为什么它不起作用。