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：

class Solution:
    def isValid(self, s: str) -> bool:
        n = len(s)
        if n == 0:
            return True
        stack = []
        for c in s:
            if c == '(' or c == '{' or c == '[':
                stack.append(c)
            elif c == ')':
                if len(stack) > 0:
                    if stack[-1] == '(':
                        stack.pop(-1)
                    else:
                        stack.append(')')
                else:
                    return False
            elif c == '}':
                if len(stack) > 0:
                    if stack[-1] == '{':
                        stack.pop(-1)
                    else:
                        stack.append('}')
                else:
                    return False
            elif c == ']':
                if len(stack) > 0:
                    if stack[-1] == '[':
                        stack.pop(-1)
                    else:
                        stack.append(']')
                else:
                    return False
        if len(stack) == 0:
            return True
        else:
            return False