22. Generate Parentheses

GenerateParentheses.java

class Solution {
    
    // 22. Generate Parentheses
    // https://leetcode.com/problems/generate-parentheses/
    
    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 == close) {
            backtrack(ans, cur.append("("), open+1, close, max);
            cur.deleteCharAt(cur.length() - 1);
            return;
        }

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