Generate Parentheses

本文介绍了一个递归算法来解决括号生成问题。给定n对括号,算法能生成所有合法的组合。通过递归尝试不同的加括号方式,确保左右括号数量平衡且结构正确。

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22. Generate Parentheses

问题描述

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

[ “((()))”,

​ “(()())”,

​ “(())()”,

​ “()(())”,

​ “()()()”]

我的解答

一个递归的解法:通过递归实现不同的加括号的尝试,如果满足条件继续递归直到符合结果结束并且返回。否则结束递归,继续尝试下一种方式。

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        vector<string> result;

        execute("", "(", n, 1, 0, result);
        return result;
    }
    void execute(string tmp, string next, int n, int l, int r, vector<string> &result) {
        tmp += next;
        if (l < r || l > n || r > n) return;
        if (l == n && r == n) {
            result.push_back(tmp);
            return;
        }
        execute(tmp, "(", n, l + 1, r, result);
        execute(tmp, ")", n, l, r + 1, result); 
    }
};
#include <cassert> /// for assert #include <iostream> /// for I/O operation #include <vector> /// for vector container /** * @brief Backtracking algorithms * @namespace backtracking */ namespace backtracking { /** * @brief generate_parentheses class */ class generate_parentheses { private: std::vector<std::string> res; ///< Contains all possible valid patterns void makeStrings(std::string str, int n, int closed, int open); public: std::vector<std::string> generate(int n); }; /** * @brief function that adds parenthesis to the string. * * @param str string build during backtracking * @param n number of pairs of parentheses * @param closed number of closed parentheses * @param open number of open parentheses */ void generate_parentheses::makeStrings(std::string str, int n, int closed, int open) { if (closed > open) // We can never have more closed than open return; if ((str.length() == 2 * n) && (closed != open)) { // closed and open must be the same return; } if (str.length() == 2 * n) { res.push_back(str); return; } makeStrings(str + ')', n, closed + 1, open); makeStrings(str + '(', n, closed, open + 1); } /** * @brief wrapper interface * * @param n number of pairs of parentheses * @return all well-formed pattern of parentheses */ std::vector<std::string> generate_parentheses::generate(int n) { backtracking::generate_parentheses::res.clear(); std::string str = "("; generate_parentheses::makeStrings(str, n, 0, 1); return res; } } // namespace backtracking /** * @brief Self-test implementations * @returns void */ static void test() { int n = 0; std::vector<std::string> patterns; backtracking::generate_parentheses p; n = 1; patterns = {{"()"}}; assert(p.generate(n) == patterns); n = 3; patterns = {{"()()()"}, {"()(())"}, {"(())()"}, {"(()())"}, {"((()))"}}; assert(p.generate(n) == patterns); n = 4; patterns = {{"()()()()"}, {"()()(())"}, {"()(())()"}, {"()(()())"}, {"()((()))"}, {"(())()()"}, {"(())(())"}, {"(()())()"}, {"(()()())"}, {"(()(()))"}, {"((()))()"}, {"((())())"}, {"((()()))"}, {"(((())))"}}; assert(p.generate(n) == patterns); std::cout << "All tests passed\n"; } /** * @brief Main function * @returns 0 on exit */ int main() { test(); // run self-test implementations return 0; } 解释一下这段代码?
03-08
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