Valid Parentheses (Java)

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.

Source

public class Solution {
    public boolean isValid(String s) {
		char a,b;
		Stack<Character> st = new Stack<Character>();		//*****
	
		for(int i = 0; i < s.length(); i++){
			a = s.charAt(i);
			if(a == '(' || a == '{' || a == '[')
				st.push(a);
			else if(a == ')' || a == '}' || a == ']'){
				if(st.size() == 0) return false;    //在pop前判空，否则RE
				b = st.pop();			//pop没有参数
				if(b == '(' && a == ')')	continue;
				if(b == '{' && a == '}')	continue;
				if(b == '[' && a == ']')	continue;
				return false;
 			}
		}
		if(st.size() == 0) return true;  //**********
		else return false;
    }
}