leetcode 20 栈

    public boolean isValid(String s) {
        Stack<Character> stack = new Stack<Character>();
        for( int i = 0 ; i < s.length() ; i ++ )
            if( s.charAt(i) == '(' || s.charAt(i) == '{' || s.charAt(i) == '[')
                stack.push(s.charAt(i));
            else{

                if( stack.size() == 0 )
                    return false;

                Character c = stack.pop();

                Character match;
                if( s.charAt(i) == ')' )
                    match = '(';
                else if( s.charAt(i) == ']' )
                    match = '[';
                else{
                    assert s.charAt(i) == '}';
                    match = '{';
                }

                if(c != match)
                    return false;
            }

        if( stack.size() != 0 )
            return false;

        return true;
    }