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：

 1 public class Solution {

 2     public boolean isValid(String s) {

 3         // Note: The Solution object is instantiated only once and is reused by each test case.

 4         // '(' - '0' :: -8

 5         // ')' - '0' :: -7

 6         

 7         // '[' - '0' :: 43

 8         // ']' - '0' :: 45

 9         

10         // '{' - '0' :: 75

11         // '}' - '0' :: 77

12         Stack<Integer> st = new Stack<Integer>();

13         for(int i = 0; i < s.length(); i ++){

14             int c = s.charAt(i) - '0';

15             if(c == -8 || c == 43 || c == 75) st.push(c);

16             else if(c == -7){

17                 if(st.size() != 0 && st.peek() == -8)st.pop();

18                 else return false;

19             }

20             else if(c == 45){

21                 if(st.size() != 0 && st.peek() == 43)st.pop();

22                 else return false;

23             }

24             else if(c == 77){

25                 if(st.size() != 0 && st.peek() == 75)st.pop();

26                 else return false;

27             }

28             else return false;

29         }

30         if(st.size() != 0) return false;

31         return true;

32     }

33 }

 

第三遍：

 1 public class Solution {

 2     public boolean isValid(String s) {

 3         LinkedList<Character> ll = new LinkedList<Character>();

 4         if(s == null || s.length() == 0) return false;

 5         for(int i = 0; i < s.length(); i ++){

 6             if(s.charAt(i) == '(' || s.charAt(i) == '[' || s.charAt(i) == '{') ll.push(s.charAt(i));

 7             else if(ll.size() == 0 || (s.charAt(i) == ')' && ll.peek() != '(') ||

 8             (s.charAt(i) == ']' && ll.peek() != '[') || (s.charAt(i) == '}' && ll.peek() != '{')) return false;

 9             else ll.pop();

10         }

11         return ll.size() == 0;

12     }

13 }