post increment vs +1 in recursive function

Question

I was working on Leetcode #22 Generate Parentheses. In one of their solutions, I noticed if we change open+1 and close+1 to ++open and ++close, the code doesn't work anymore. I thought we could still use pre-increment in recursion, so I don't understand what makes the difference here.

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> ans = new ArrayList();
        backtrack(ans, new StringBuilder(), 0, 0, n);
        return ans;
    }

    public void backtrack(List<String> ans, StringBuilder cur, int open, int close, int max){
        if (cur.length() == max * 2) {
            ans.add(cur.toString());
            return;
        }

        if (open < max) {
            cur.append("(");
            backtrack(ans, cur, open+1, close, max);
            cur.deleteCharAt(cur.length() - 1);
        }
        if (close < open) {
            cur.append(")");
            backtrack(ans, cur, open, close+1, max);
            cur.deleteCharAt(cur.length() - 1);
        }
    }
}

Answer 1

The difference between passing value using ++open and open+1 is that in the former, the value of the variable open is changed, while in the latter it is not. That is,

• If open is, say 2, after ++open, the value of open is 3
• However, when you do open+1, you only pass 3 but the value of open is still 2. So, the next loop gets the same value.

To better understand, the following is equivalent to ++open:

backtrack(ans, cur, (open = open+1), close, max);

In the above code, the bracketed expression is evaluated to set the value of open to open+1 before being passed to the method.