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.

第一遍：

public class Solution {
    public boolean isValid(String s) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        // '(' - '0' :: -8
        // ')' - '0' :: -7
        
        // '[' - '0' :: 43
        // ']' - '0' :: 45
        
        // '{' - '0' :: 75
        // '}' - '0' :: 77
        Stack<Integer> st = new Stack<Integer>();
        for(int i = 0; i < s.length(); i ++){
            int c = s.charAt(i) - '0';
            if(c == -8 || c == 43 || c == 75) st.push(c);
            else if(c == -7){
                if(st.size() != 0 && st.peek() == -8)st.pop();
                else return false;
            }
            else if(c == 45){
                if(st.size() != 0 && st.peek() == 43)st.pop();
                else return false;
            }
            else if(c == 77){
                if(st.size() != 0 && st.peek() == 75)st.pop();
                else return false;
            }
            else return false;
        }
        if(st.size() != 0) return false;
        return true;
    }
}

第三遍：

public class Solution {
    public boolean isValid(String s) {
        LinkedList<Character> ll = new LinkedList<Character>();
        if(s == null || s.length() == 0) return false;
        for(int i = 0; i < s.length(); i ++){
            if(s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') ll.push(s.charAt(i));
            else if(ll.size() == 0 || (s.charAt(i) == ')' && ll.peek() != '(') ||
            (s.charAt(i) == ']' && ll.peek() != '[') || (s.charAt(i) == '}' && ll.peek() != '{')) return false;
            else ll.pop();
        }
        return ll.size() == 0;
    }
}