[LeetCode] 20. Valid Parentheses

Given a string containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

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public class Solution {
    public boolean isValid(String s) {
        
        int len = s.length();
        
        if(0 == len)
            return true;
        
        // can not use char in template
        // Use queue instead of stack at first ........, can't find the bug even.
        Stack pq = new Stack();

        /*
        pq.peek() == new Character('[') 
        OK.
        pq.peek() == '['
        '[' == pq.peek()
        null return can not compare with primitive char
        null pointer execption
        */
        

        for(int i = 0; i < len; ++i){
            
            char c = s.charAt(i);
            
            if(c =='(' || c =='{' || c =='['){
                pq.push(c);
            // The peek operation is different from queue to stack. We can peek empty queue, but not for stack. 
            }else if(pq.empty()){
                return false;
            }else{
                if(c == ')')
                    // use poll instead of peek here first, not working for input "]", empty queue, must peek before poll
                    if(pq.peek().charValue() == '(') pq.pop(); else return false;
                if(c == '}')
                    if(pq.peek().charValue() == '{') pq.pop(); else return false;
                if(c == ']')
                    if(pq.peek().charValue() == '[') pq.pop(); else return false;
            }
        }
        
        // shouldn't return true here. pq must be empty
        return pq.empty();
        
    }
}