Leetcode: Valid Palindromic

Question

Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

For example,
“A man, a plan, a canal: Panama” is a palindrome.
“race a car” is not a palindrome.

Note:
Have you consider that the string might be empty? This is a good question to ask during an interview.

For the purpose of this problem, we define empty string as valid palindrome.

Hide Tags Two Pointers String

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Analysis

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Soluion

v1, Space Complexity O(n)

class Solution:
    # @param {string} s
    # @return {boolean}
    def isPalindrome(self, s):
        if s=='':
            return True

        s = [temp.upper() for temp in list(s)]
        s = [val for val in s if ord(val) in range(65,91) or ord(val) in range(48,58)]

        s_ori = s[:]
        s.reverse()

        return s_ori==s

v2, Space Complexity O(1)

Mistake Taken

the incorrect code is

class Solution:
    # @param {string} s
    # @return {boolean}
    def isPalindrome(self, s):
        if s=='':
            return True

        l,r = 0,len(s)-1
        while(l<=r):
            while not self.isvalid(s[l]) and l<len(s):
                l += 1

            while not self.isvalid(s[r]) and r<len(s):
                r -= 1

            if l>r or not self.issame(s[l],s[r]):
                return False


        return True

    def isvalid(self,p):
        p = p.upper()
        return True if ord(p) in range(65,91) or ord(p) in range(48,58) else False

    def issame(self,p,q):
        p,q = p.upper(), q.upper()
        return True if p==q else False

However, if the input is ″ ″, the output is False, but the expected output is True.

The solution is that in while(l<=r), moving one step(for l and r) each time. If input contains multiple ′ ′s, while(l<=r) will return nothing.

Correct code

class Solution:
    # @param {string} s
    # @return {boolean}
    def isPalindrome(self, s):
        if s=='':
            return True

        l,r = 0,len(s)-1
        while(l<=r):
            if not self.isvalid(s[l]):
                l += 1
                continue

            if not self.isvalid(s[r]):
                r -= 1
                continue

            if not self.issame(s[l],s[r]):
                return False

            l += 1
            r -= 1

        return True

    def isvalid(self,p):
        p = p.upper()
        return True if ord(p) in range(65,91) or ord(p) in range(48,58) else False

    def issame(self,p,q):
        p,q = p.upper(), q.upper()
        return True if p==q else False