题目描述：
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.
在一个只包含"()[]{}"的输入中，判断括号的合法性。
思路：
栈的基本使用。
实现代码：public class Solution {
public bool IsValid(string s)
{
var stack = new Stack<char>();
for(var i = 0;i < s.Length; i++){
switch(s[i]){
case '{':
case '[':
case '(':
stack.Push(s[i]);
break;
case '}':
if(stack.Count > 0 && stack.Peek() == '{'){
stack.Pop();
}
else{
return false;
}
break;
case ']':
if(stack.Count > 0 && stack.Peek() == '['){
stack.Pop();
}
else{
return false;
}
break;
case ')':
if(stack.Count > 0 && stack.Peek() == '('){
stack.Pop();
}
else{
return false;
}
break;
default :
throw new ArgumentException(s);
}
}
return stack.Count == 0;
}
}