678. Valid Parenthesis String

Given a string containing only three types of characters: '(', ')' and '*', write a function to check whether this string is valid. We define the validity of a string by these rules:
Any left parenthesis 

'('

 must have a corresponding right parenthesis 

')'

.
Any right parenthesis 

')'

 must have a corresponding left parenthesis 

'('

.
Left parenthesis 

'('

 must go before the corresponding right parenthesis 

')'

.

'*'

 could be treated as a single right parenthesis 

')'

 or
a single left parenthesis 

'('

 or an empty string.
An empty string is also valid.

Example 1:

Input: "()"
Output: True

Example 2:

Input: "(*)"
Output: True

Example 3:

Input: "(*))"
Output: True

class Solution {
public boolean checkValidString(String s) {
Stack<Integer> st1 = new Stack<>();
Stack<Integer> st2 = new Stack<>();
for (int i = 0; i < s.length(); ++i) {
char c = s.charAt(i);
if (c == '(')
st1.add(i);
else if (c == '*')
st2.add(i);
else {
if (!st1.isEmpty())
st1.pop();
else if (!st2.isEmpty())
st2.pop();
else
return false;
}
}
while (!st1.isEmpty()) {
int n1 = st1.pop();
if (st2.isEmpty())
return false;
int n2 = st2.pop();
if (n1 > n2)
return false;
}
return true;
}
}