20 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.

Solution:

class Solution {
public:
    bool isValid(string s) {
        int n = s.size();
        if ( n == 0 ) return true;
        stack<char> stack;
        for ( int i = 0; i <= n-1; i++ ) {
            if ( s[i] == '(' or s[i] == '[' or s[i] == '{') stack.push(s[i]);
            if ( s[i] == ')' or s[i] == ']' or s[i] == '}') {
                if ( s[i] == ')' and (stack.empty() or stack.top() != '(' )) return false;
                if ( s[i] == ']' and (stack.empty() or stack.top() != '[' )) return false;
                if ( s[i] == '}' and (stack.empty() or stack.top() != '{' )) return false;
                stack.pop();
            }
        }
        return stack.empty();
    }
};

Notes: note the difference between "" and ''.

char* ab = "123" ; //a string
char* cd = 'a' ;   //a character.

A cleaner code using HashMap

class Solution {
public:
    bool isValid(string s) {
        unordered_map<char, char> map;
        map['('] = ')';
        map['['] = ']';
        map['{'] = '}';
        int n = s.size();
        stack<char> left;
        for ( int i = 0; i < n; i++ ) {
            if ( map.find(s[i]) != map.end() ) left.push(s[i]);
            else {
                if ( left.empty() ) return false;
                if ( map[left.top()] != s[i] ) return false;
                left.pop();
            }
        }
        return left.empty();
    }
};