20. Valid Parentheses括号匹配

class Solution {
public:
    bool isValid(string s) {
        stack<char> parent;
        for(char c:s){
            if(c ==‘[‘||c == ‘(‘ || c == ‘{‘) parent.push(c);  //左括号进栈
            else if(parent.empty()) return false;   
            else{                                              //有右括号且与栈顶匹配，则栈顶元素出栈
                if(c ==‘]‘ && parent.top() == ‘[‘) parent.pop();
                else if(c ==‘}‘ && parent.top() == ‘{‘) parent.pop();
                else if(c ==‘)‘ && parent.top() == ‘(‘) parent.pop();
                else return false;
            }
        }
        if(parent.empty()) return true;
        else return false;
    }
    
};

public boolean isValid(String s) {
    Stack<Character> stack = new Stack<Character>();
    for (char c : s.toCharArray()) {
        if (c == ‘(‘)                                 //遇到左右括号，使括号进栈
            stack.push(‘)‘);
        else if (c == ‘{‘)
            stack.push(‘}‘);
        else if (c == ‘[‘)
            stack.push(‘]‘);
        else if (stack.isEmpty() || stack.pop() != c)  //遇到非右括号，若栈空，返回false。否则弹出栈顶元素，看其是否与当前元素相等，否则错误
            return false;
    }
    return stack.isEmpty();
}

 bool isValid(string s) {
        stack<char> paren;
        for (char& c : s) {
            switch (c) {
                case ‘(‘: 
                case ‘{‘: 
                case ‘[‘: paren.push(c); break;
                case ‘)‘: if (paren.empty() || paren.top()!=‘(‘) return false; else paren.pop(); break;
                case ‘}‘: if (paren.empty() || paren.top()!=‘{‘) return false; else paren.pop(); break;
                case ‘]‘: if (paren.empty() || paren.top()!=‘[‘) return false; else paren.pop(); break;
                default: ; // pass
            }
        }
        return paren.empty() ;
    }