题目：
给定一个字符串str,判断是不是整体有效的括号字符串
举例：
str = "()" return true; stre = "()()" return true;
str = "())" return false; str = "()a()" return false;
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代码：
package com.chenyu.string.cn;
public class IsValid {
public static void main(String[] args) {
String s1 = "()";
String s2 = "()(()";
String s3 = "()(4)";
System.out.println("result1:" + isValid(s1));
System.out.println("result2:" + isValid(s2));
System.out.println("result3:" + isValid(s3));
}
public static boolean isValid(String s) {
if (s == null || "".equals(s)) {
return false;
}
char[] chars = s.toCharArray();
int count = 0;
for (int i = 0; i < chars.length; i++) {
i
