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.

---

主要思想还是盏。

 public static boolean isValid(String s) {
     HashMap<Character, Character> map = new HashMap<Character, Character>();
     map.put('(', ')');
     map.put('[', ']');
     map.put('{', '}');

     Stack<Character> stack = new Stack<Character>();

     for (int i = 0; i < s.length(); i++) {
         char curr = s.charAt(i);

         if (map.keySet().contains(curr)) {
             stack.push(curr);
         } else if (map.values().contains(curr)) {
             if (!stack.empty() && map.get(stack.peek()) == curr) {
                 stack.pop();
             } else {
                 return false;
             }
         }
     }

     return stack.empty();
 }

 public class S20 {  

     public static void main(String[] args) {  

     }  

     // 用stack来检查
     public boolean isValid(String s) {
         Stack<Character> stack = new Stack<Character>();
         for(int i=0; i<s.length(); i++){
             char c = s.charAt(i);
             // 如果遇到前括号就压入栈
             if(c=='(' || c=='[' || c=='{'){
                 stack.push(c);
             }else if(c==')' || c==']' || c=='}'){       // 遇到后括号就出栈
                 if(stack.size() == 0){  //  说明后括号太多了
                     return false;
                 }
                 char cpop = stack.pop();
                 if(cpop=='(' && c==')'){
                     continue;
                 }else if(cpop=='[' && c==']'){
                     continue;
                 }else if(cpop=='{' && c=='}'){
                     continue;
                 }
                 return false;
             }
         }
         return stack.size()==0;
     }  

 }