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:

1. Any left parenthesis '(' must have a corresponding right parenthesis ')'.
2. Any right parenthesis ')' must have a corresponding left parenthesis '('.
3. Left parenthesis '(' must go before the corresponding right parenthesis ')'.
4. '*' could be treated as a single right parenthesis ')' or a single left parenthesis '(' or an empty string.
5. 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;    }}