LeetCode Online Judge 题目C# 练习 - 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.

 1         public static bool ValidParentheses(string s)

 2         {

 3             Stack<char> stack = new Stack<char>();

 4 

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

 6             {

 7                 if (s[i] == '(' || s[i] == '[' || s[i] == '{')

 8                     stack.Push(s[i]);

 9                 else

10                 {

11                     // begin with ')', ']' or '}'?

12                     if (stack.Count == 0)

13                         return false;

14                     

15                     if (s[i] == ')')

16                     {

17                         if (stack.Peek() == '(')

18                         {

19                             stack.Pop();

20                         }

21                         else

22                             return false;

23                     }

24                     else if (s[i] == ']')

25                     {

26                         if (stack.Peek() == '[')

27                         {

28                             stack.Pop();

29                         }

30                         else

31                             return false;

32                     }

33                     else if (s[i] == '}')

34                     {

35                         if (stack.Peek() == '{')

36                         {

37                             stack.Pop();

38                         }

39                         else

40                             return false;

41                     }

42                 }

43             }

44 

45             return stack.Count == 0;

46         }

代码分析：

　　比较简单，一个Stack， 左括号（大，中，小）往里push， 右括号，pop出来，不match return false， 

　　最后要看stack里是否空，不空则左括号多了，return false。