Q1:Valid Parentheses

Question：

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.

MyAnswer 1 (C++)：

 class Solution {
 public:
     bool isValid(string s) {
         char stack[s.size()];
         int i = ;
         int j = ;

         //cout << s;

         if(s[j] == ')' || s[j] == ']' || s[j] =='}')
             return false;
         else if(s[j] == '\0')
             return true;
         else
             stack[i++] = s[j++];

         while()
         {
             switch(s[j])
             {
                 case '\0':
                     if(i == )
                         return true;
                     else
                         return false;
                 case ')':
                     if(stack[i-] != '(')
                         return false;
                     else
                     {
                         i -= ;
                         j += ;
                     }
                     break;
                 case ']':
                     if(stack[i-] != '[')
                         return false;
                     else
                     {
                         i -= ;
                         j += ;
                     }
                     break;
                 case '}':
                     if(stack[i-] != '{')
                         return false;
                     else
                     {
                         i -= ;
                         j += ;
                     }
                     break;
                 default:
                     stack[i++] = s[j++];
                     break;
             }
         }
     }
 };

数组模拟栈，匹配到括号则出栈，否则入栈

MyAnswer 2 (Java):

 public class Solution {
     public boolean isValid(String s) {
         ArrayList<Character> stack = new ArrayList<Character>();
         int i = 0;
         for(i = 0; i < s.length(); i++){
                 switch(s.charAt(i)){
                         case ')':
                                 if(stack.isEmpty() || stack.get(stack.size()-1) != '(')
                                         return false;
                                 else
                                         stack.remove(stack.size()-1);
                                 break;
                         case ']':
                                 if(stack.isEmpty() || stack.get(stack.size()-1) != '[')
                                         return false;
                                 else
                                         stack.remove(stack.size()-1);
                                 break;
                         case '}':
                                 if(stack.isEmpty() || stack.get(stack.size()-1) != '{')
                                         return false;
                                 else
                                         stack.remove(stack.size()-1);
                                 break;
                         default:
                                 stack.add(s.charAt(i));
                                 break;
                 }
         }
         if(stack.isEmpty())
                 return true;
         else
                 return false;
     }
 }