leetcode Question 120: Valid Parentheses

Valid Parentheses

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.

Analysis:
Idea is not complex.
Use a stack to store the chars, scan from the 1st to the last char in string s.
( [ { are free to push in the stack.
When meets ) if stack top is (, then pop (.
When meets ] if stack top is [, then pop [.
When meets } if stack top is {, then pop {.
Otherwise return false

In the end, if the stack is empty, return true. (to handle "()(" case )


Code:

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]);}
            else{   
                if (st.empty()){return false;}
                if ((s[i]==')') && (st.top()!='(')) {return false;}
                if ((s[i]=='{') && (st.top()!='}')) {return false;}
                if ((s[i]=='[') && (st.top()!=']')) {return false;}
                st.pop();
            }
            
        }
        return st.empty();
    }
};