leetcode20

public class Solution {
    Stack<char> S = new Stack<char>();

        public bool IsValid(string s)
        {
            foreach (var c in s)
            {
                if (c == '(' || c == '[' || c == '{')
                {
                    S.Push(c);
                }
                else if(c == ')')
                {
                    if (S.Count > )
                    {
                        var k = S.Peek();
                        if (k == '(')
                        {
                            S.Pop();
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
                else if (c == ']')
                {
                    if (S.Count > )
                    {
                        var k = S.Peek();
                        if (k == '[')
                        {
                            S.Pop();
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
                else if (c == '}')
                {
                    if (S.Count > )
                    {
                        var k = S.Peek();
                        if (k == '{')
                        {
                            S.Pop();
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
            }
            if (S.Count > )
            {
                return false;
            }
            else
            {
                return true;
            }
        }
}

https://leetcode.com/problems/valid-parentheses/#/description

补充一个python的实现：

 class Solution:
     def isValid(self, s: str) -> bool:
         n = len(s)
         if n == :
             return True
         stack = []
         for c in s:
             if c == '(' or c == '{' or c == '[':
                 stack.append(c)
             elif c == ')':
                 if len(stack) > :
                     if stack[-] == '(':
                         stack.pop(-)
                     else:
                         stack.append(')')
                 else:
                     return False
             elif c == '}':
                 if len(stack) > :
                     if stack[-] == '{':
                         stack.pop(-)
                     else:
                         stack.append('}')
                 else:
                     return False
             elif c == ']':
                 if len(stack) > :
                     if stack[-] == '[':
                         stack.pop(-)
                     else:
                         stack.append(']')
                 else:
                     return False
         if len(stack) == :
             return True
         else:
             return False