February 10, 2019

22. Generate Parentheses

Backtracking
Time = O(2^n)
Space = O(n)

Java 用 StringBuilder：

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res = new ArrayList<>();
        StringBuilder sb = new StringBuilder();
        parenthesisHelper(0, 0, n, sb, res);
        return res;
    }
    
    private void parenthesisHelper(int left, int right, int n, StringBuilder sb, List<String> res) {
        if (left == n && right == n) {
            res.add(sb.toString());
            return;
        }
        if (left < n) {
            parenthesisHelper(left + 1, right, n, sb.append("("), res);
            sb.deleteCharAt(sb.length() - 1);
        }
        if (right < left) {
            parenthesisHelper(left, right + 1, n, sb.append(")"), res);
            sb.deleteCharAt(sb.length() - 1);
        }
    }
}

Java 用 String：

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> res = new ArrayList<>();
        parenthesisHelper(0, 0, n, "", res);
        return res;
    }
    
    private void parenthesisHelper(int left, int right, int n, String s, List<String> res) {
        if (left == n && right == n) {
            res.add(s);
            return;
        }
        if (left < n) {
            parenthesisHelper(left + 1, right, n, s + "(", res);
        }
        if (right < left) {
            parenthesisHelper(left, right + 1, n, s + ")", res);
        }
    }
}

Python：

class Solution:
    def generateParenthesis(self, n: 'int') -> 'List[str]':
        res = []
        self.helper(0, 0, n, "", res)
        return res
    
    def helper(self, left, right, n, cur, res):
        if left == n and right == n:
            res.append(cur)
            return
        if left < n:
            self.helper(left + 1, right, n, cur + "(", res)
        if right < n and left > right:
            self.helper(left, right + 1, n, cur + ")", res)