public class Solution {

    /**

     * @param s: A string

     * @return: whether the string is a valid parentheses

     */

    public boolean isValidParentheses(String s) {

        // write your code here

        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{ //pop前要先检查栈是不是空的

                if(!stack.isEmpty()){

                    switch(s.charAt(i)){

                    case ')':

                        if(!stack.pop().equals('(')){

                            return false;

                        }

                        break;

                    case ']':

                        if(!stack.pop().equals('[')){

                            return false;

                        }

                        break;

                    case '}':

                        if(!stack.pop().equals('{')){

                            return false;

                        }

                        break;

                    }

                }else{

                    return false;

                }

            }

        }

        if(stack.isEmpty()){

            return true;

        }else{

            return false;

        }

    }

}