20. Valid Parentheses(C++)

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

Solution :

class Solution {
public:
    bool isValid(string s) {
        if(s.size()<2) return false;
        stack<char> stk;
        for(int i=0;i<s.size();i++){
            if((s[i]=='(')||(s[i]=='[')||(s[i]=='{')) stk.push(s[i]);
            if((s[i]=='}')||(s[i]==']')||(s[i]==')')){
                if(stk.empty()) return false;
                char c=stk.top();
                stk.pop();
                switch (s[i]){
                    case '}': if(c!='{') return false; break;
                    case ')': if(c!='(') return false; break;
                    case ']': if(c!='[') return false; break;
                }
            }
        }
        if(!stk.empty()) return false;
        return true;
    }
};

better solution:

class Solution {
public:
    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() ;
    }
};