301. delete invalid brackets

Preface

C++ It's a high-level programming language , from C Language expansion and upgrading , As early as 1979 By Benjani · Strauss LUP is AT&T Developed by Bell studio .
C++ Both can be carried out C Process programming of language , It can also be used for object-based programming characterized by abstract data types , It can also carry out object-oriented programming characterized by inheritance and polymorphism .C++ Good at object-oriented programming at the same time , You can also do process based programming .
C++ Have the practical characteristics of computer operation , At the same time, it is also committed to improving the programming quality of large-scale programs and the problem description ability of programming languages .

Java Is an object-oriented programming language , Not only absorbed C++ The advantages of language , It's abandoned C++ The incomprehensible inheritance in 、 Concepts such as pointer , therefore Java Language has two characteristics: powerful and easy to use .Java As the representative of static object-oriented programming language , Excellent implementation of object-oriented theory , Allow programmers to do complex programming in an elegant way of thinking .
Java It's simple 、 object-oriented 、 Distributed 、 Robustness, 、 Security 、 Platform independence and portability 、 Multithreading 、 Dynamic and so on .Java Can write desktop applications 、Web Applications 、 Distributed system and embedded system applications, etc .

Python By Guido of the Dutch Society for mathematical and computer science research · Van rosum On 1990 It was designed in the early 's , As a course called ABC A substitute for language .Python Provides efficient advanced data structure , It's also a simple and effective way of object-oriented programming .Python Syntax and dynamic types , And the nature of interpretative language , Make it a programming language for scripting and rapid application development on most platforms , With the continuous update of the version and the addition of new language features , Gradually used for independent 、 Development of large projects .
Python The interpreter is easy to extend , have access to C Language or C++（ Or something else can be done through C Calling language ） Expand new functions and data types .Python It can also be used as an extensible programming language in customizable software .Python Rich library of standards , Provides source code or machine code for each major system platform .
2021 year 10 month , Compiler for language popularity index Tiobe take Python Crowned the most popular programming language ,20 Put it in... For the first time in years Java、C and JavaScript above .

describe

Here's a string of parentheses and letters s , Remove the minimum number of invalid parentheses , Make the input string valid .

Returns all possible results . You can press In any order return .

Example 1：

Input ：s = "()())()"
Output ：["(())()","()()()"]

Example 2：

Input ：s = "(a)())()"
Output ：["(a())()","(a)()()"]

Example 3：

Input ：s = ")("
Output ：[""]

class Solution {
    private List<String> res = new ArrayList<String>();

    public List<String> removeInvalidParentheses(String s) {
        int lremove = 0;
        int rremove = 0;

        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) == '(') {
                lremove++;
            } else if (s.charAt(i) == ')') {
                if (lremove == 0) {
                    rremove++;
                } else {
                    lremove--;
                }
            }
        }
        helper(s, 0, lremove, rremove);

        return res;
    }

    private void helper(String str, int start, int lremove, int rremove) {
        if (lremove == 0 && rremove == 0) {
            if (isValid(str)) {
                res.add(str);
            }
            return;
        }

        for (int i = start; i < str.length(); i++) {
            if (i != start && str.charAt(i) == str.charAt(i - 1)) {
                continue;
            }
            //  If the remaining characters cannot meet the number of characters to be removed , Go straight back to 
            if (lremove + rremove > str.length() - i) {
                return;
            }
            //  Try to remove an open parenthesis 
            if (lremove > 0 && str.charAt(i) == '(') {
                helper(str.substring(0, i) + str.substring(i + 1), i, lremove - 1, rremove);
            }
            //  Try to remove a closing bracket 
            if (rremove > 0 && str.charAt(i) == ')') {
                helper(str.substring(0, i) + str.substring(i + 1), i, lremove, rremove - 1);
            }
        }
    }

    private boolean isValid(String str) {
        int cnt = 0;
        for (int i = 0; i < str.length(); i++) {
            if (str.charAt(i) == '(') {
                cnt++;
            } else if (str.charAt(i) == ')') {
                cnt--;
                if (cnt < 0) {
                    return false;
                }
            }
        }

        return cnt == 0;
    }
}