Leetcode 22) Generate Parentheses

22 Jul 2021 in Algorithms

내 답안

class Solution {

    public List<String> generateParenthesis(int n) {
        return recursive(0,n);
    }

    public List<String> recursive(int close, int left){
        List<String> cur = new ArrayList<>();
        if(close ==0 && left==0){
            cur.add("");
            return cur;
        } 

        List<String> prev;

        if(left!=0){
            prev =recursive(close+1,left-1);
            for(String i : prev ){
                cur.add("("+i);
            }
            if(close==0){
                return cur;
            }
        }

        prev =recursive(close-1,left);
        for(String i : prev ){
            cur.add(")"+i);
        }
        return cur;

    }
}

다른 답안

class Solution {
    //O(n) space, O 4 to power n
    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);
        }
    }
}

오답

• cur.add(“”); 이걸 안해줘서 null이 됐고, 다음 loop문들 다 못돌아서 null값 뜸!

public List<String> recursive(int close, int left){
    List<String> cur = new ArrayList<>();
    if(close ==0 && left==0){
        //cur.add("");
        return cur;
    }