lintcode 有效的括号序列

心血来潮，半夜做了一道Lintcode的题目，调试完睡觉，欢迎大家批评指正。

public boolean isValidParentheses(String s) {        // Write your code here        Stack stack=new Stack();        if(s.length()==0){            return true;        }else if(s.length()==1){            return false;        }for(int i=0;i<s.length();i++){if(s.charAt(i)=='('||s.charAt(i)=='['||s.charAt(i)=='{'){stack.add(s.charAt(i));}else if(s.charAt(i)==')'){if(!stack.isEmpty()&&stack.peek().equals('(')){stack.pop();}else{return false;}}else if(s.charAt(i)==']'){if(!stack.isEmpty()&&stack.peek().equals('[')){stack.pop();}else{return false;}}else if(s.charAt(i)=='}'){if(!stack.isEmpty()&&stack.peek().equals('{')){stack.pop();}else{return false;}}}return stack.isEmpty();    }