学步园

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.

思路：

典型的堆栈数据结构问题。在碰到左括号的时候入栈，右括号的时候出栈。最后栈如果是空，则表明左右括号对称。

题解：

class Solution {
public:
    char counterPart(const char ch)
    {
        switch(ch)
        {
            case '{':
                return '}';
            case '[':
                return ']';
            case '(':
                return ')';
            default:
                return 0;
        }
    }

    bool isValid(string s) {
        stack<char> parenthesis;
        
        for(auto ch: s)
            if (ch == '(' || ch == '[' || ch == '{')
                parenthesis.push(ch);
            else
                if (!parenthesis.empty() && ch == counterPart(parenthesis.top()))
                    parenthesis.pop();
                else
                    return false;
        
        return parenthesis.empty();
    }
};