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++)：

 1 class Solution {
 2 public:
 3     bool isValid(string s) {
 4         char stack[s.size()];
 5         int i = 0;
 6         int j = 0;
 7         
 8         //cout << s;
 9         
10         if(s[j] == ')' || s[j] == ']' || s[j] =='}')
11             return false;
12         else if(s[j] == '')
13             return true;
14         else
15             stack[i++] = s[j++];
16     
17         while(1)
18         {
19             switch(s[j])
20             {
21                 case '':
22                     if(i == 0)
23                         return true;
24                     else 
25                         return false;
26                 case ')':
27                     if(stack[i-1] != '(')
28                         return false;
29                     else
30                     {
31                         i -= 1;
32                         j += 1;
33                     }
34                     break;
35                 case ']':
36                     if(stack[i-1] != '[')
37                         return false;
38                     else
39                     {
40                         i -= 1;
41                         j += 1;
42                     }
43                     break;
44                 case '}':
45                     if(stack[i-1] != '{')
46                         return false;
47                     else
48                     {
49                         i -= 1;
50                         j += 1;
51                     }
52                     break;
53                 default:
54                     stack[i++] = s[j++];
55                     break;
56             }
57         }
58     }
59 };

数组模拟栈，匹配到括号则出栈，否则入栈

MyAnswer 2 (Java):

 1 public class Solution {
 2     public boolean isValid(String s) {
 3         ArrayList<Character> stack = new ArrayList<Character>();
 4         int i = 0;
 5         for(i = 0; i < s.length(); i++){
 6                 switch(s.charAt(i)){
 7                         case ')':
 8                                 if(stack.isEmpty() || stack.get(stack.size()-1) != '(')
 9                                         return false;
10                                 else
11                                         stack.remove(stack.size()-1);
12                                 break;
13                         case ']':
14                                 if(stack.isEmpty() || stack.get(stack.size()-1) != '[')
15                                         return false;
16                                 else
17                                         stack.remove(stack.size()-1);
18                                 break;
19                         case '}':
20                                 if(stack.isEmpty() || stack.get(stack.size()-1) != '{')
21                                         return false;
22                                 else
23                                         stack.remove(stack.size()-1);
24                                 break;
25                         default:
26                                 stack.add(s.charAt(i));
27                                 break;
28                 }
29         }
30         if(stack.isEmpty())
31                 return true;
32         else
33                 return false;
34     }
35 }