For MultiTek

Start to test at 2018 Feb 28 8:00

20. Valid Parentheses

class Solution {
    /*if c = '(' or '{' or '[', return true; otherwise, return false*/
    bool isLeftOperator(char c) {
        return ((c == '{') || (c == '[') || (c == '('));
    }
    bool isMatch(char c1, char c2) {
        if (c1 == '}') 
            return (c2 == '{');
        if (c1 == ']') 
            return (c2 == '[');
        if (c1 == ')') 
            return (c2 =='(');
        return false;
    }
public:
    // '(', ')', '{', '}', '[' and ']', 
    bool isValid(string s) {
        /*use a stack to store the operators*/
        stack<char> stack;
        char now, c;
        for (int i = 0; i < s.length(); i++) {
            now = s.at(i);
            if (isLeftOperator(now)) {
                stack.push(now);
            } else {
                /*boundary condition*/
                if (stack.empty())
                    return false;
                char c = stack.top();
                /*try to match*/
                if (!isMatch(now, c)) {
                    return false;
                }
                stack.pop();
            }
        }
        if (stack.empty()) {
            return true;
        } else {
            return false;
        }
    }
};

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