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 > 0)
                    {
                        var k = S.Peek();
                        if (k == '(')
                        {
                            S.Pop();
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
                else if (c == ']')
                {
                    if (S.Count > 0)
                    {
                        var k = S.Peek();
                        if (k == '[')
                        {
                            S.Pop();
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
                else if (c == '}')
                {
                    if (S.Count > 0)
                    {
                        var k = S.Peek();
                        if (k == '{')
                        {
                            S.Pop();
                        }
                        else
                        {
                            return false;
                        }
                    }
                    else
                    {
                        return false;
                    }
                }
            }
            if (S.Count > 0)
            {
                return false;
            }
            else
            {
                return true;
            }
        }
}

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

补充一个python的实现：

 1 class Solution:
 2     def isValid(self, s: str) -> bool:
 3         n = len(s)
 4         if n == 0:
 5             return True
 6         stack = []
 7         for c in s:
 8             if c == '(' or c == '{' or c == '[':
 9                 stack.append(c)
10             elif c == ')':
11                 if len(stack) > 0:
12                     if stack[-1] == '(':
13                         stack.pop(-1)
14                     else:
15                         stack.append(')')
16                 else:
17                     return False
18             elif c == '}':
19                 if len(stack) > 0:
20                     if stack[-1] == '{':
21                         stack.pop(-1)
22                     else:
23                         stack.append('}')
24                 else:
25                     return False
26             elif c == ']':
27                 if len(stack) > 0:
28                     if stack[-1] == '[':
29                         stack.pop(-1)
30                     else:
31                         stack.append(']')
32                 else:
33                     return False
34         if len(stack) == 0:
35             return True
36         else:
37             return False