Question ：

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.

Anwser 1 ：

Stack

class Solution {
public:
    bool isValid(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<char> st;
        for(int i = 0; i < s.size(); i++)
        {
            if(s[i] == '(' || s[i] == '{' || s[i] == '['){
                st.push(s[i]);
            }
               
            if(s[i] == ')')
            {
                if(st.empty() || st.top() != '(')
                   return false;
                st.pop();
            }
            if(s[i] == '}')
            {
                if(st.empty() || st.top() != '{')
                   return false;
                st.pop();
            }
            if(s[i] == ']')
            {
                if(st.empty() || st.top() != '[')
                   return false;
                st.pop();
            }            
        }
        if(st.empty() == 0)
           return false;
           
        return true;
    }
};

Anwser 2 ：

class Solution {
public:
    bool isValid(string s) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<char> st;
        for (int i = 0; i < s.size(); i++) {
            char c = s[i];
            if (isLeft(c)) {    // push
                st.push(c);
            } else {
                if (st.empty()) {
                    return false;
                }
                char d = st.top();      // pop
                st.pop();
                if (!match(d,  c)) {
                    return false;
                }
            }
        }
 
        if (st.empty()) {
            return true;
        }
        else {
            return false;
        }
    }
 
    bool isLeft(char c) {
        return c == '{' || c == '[' || c == '(';
    }
    
    bool match(char c, char d) {
        return (c == '(' && d == ')') || (c == '[' && d == ']') || (c == '{' && d == '}');
    }
};