最長有效括號【動態規劃】【模擬棧】

最長有效括號

使用棧操作

class Solution(object):
    def longestValidParentheses(self, s):
        maxlen=0
        stack=[-1]
        for i in range(len(s)):
            if s[i]=="(":
                stack.append(i)
            else :
                del stack[-1]
                if not len(stack)==0:
                    maxlen=max(maxlen,i-stack[-1])
                else :
                    stack.append(i)
        return maxlen   

暴力法枚舉起始位置，超時

class Solution(object):
    def find(self,s):
        stack=[]
        for i in range(len(s)):
            if s[i]=="(":
                stack.append("(")
            elif s[i]==")":
                if len(stack)==0 or stack[-1]==")" :
                    return False
                else :
                    del stack[-1]
            else:
                return False
        if not len(stack)==0 :
            return False
        else :
            return True
    def longestValidParentheses(self, s):
        maxlen=0
        for i in range(len(s)):
            for j in range(i+2,len(s)+1,2):
               # print(i,j,self.find(s[i:j]),s[i:j])
                if self.find(s[i:j])==True :
                    maxlen=max(maxlen,j-i)
        return maxlen  

動態規劃解決

class Solution(object):
    def longestValidParentheses(self, s):
        maxlen=0
        dp=[0]*(len(s)+1)
        for i in range(1,len(s)):
            if s[i]==")" and s[i-1]=="(":
                dp[i]=dp[i-2]+2
            if s[i]==")" and s[i-1]==")":
                if i-dp[i-1]>0 and s[i-dp[i-1]-1]=="(":
                    dp[i]=dp[i-dp[i-1]-2]+2+dp[i-1]
        return max(dp)   

設立左右雙指針

class Solution(object):
    def longestValidParentheses(self, s):
        maxlen,left,right=0,0,0
        for i in range(len(s)):
            if s[i]=="(":
                left+=1
            else :
                right+=1
            if right==left:
                maxlen=max(maxlen,right*2)
            elif right>left:
                right=left=0
        left,right=0,0
        for i in reversed(range(len(s))):
            if s[i]==")":
                left+=1
            else :
                right+=1
            if right==left:
                maxlen=max(maxlen,right*2)
            elif right>left:
                right=left=0 
        return maxlen