import java.util.*;
public class Solution {
public boolean isValid(String s) {
if(s==null)
return true;
if(s.length()%2==1)
return false;
Stack stack=new Stack();
for(int i=0;i 
import java.util.*;
public class Solution {
public boolean isValid(String s) {
if(s==null)
return true;
if(s.length()%2==1)
return false;
Stack stack=new Stack();
for(int i=0;i