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Python: search longest palindromes within a word and palindromes within a word/string

由 北城余情 提交于 2019-11-27 22:38:32

Your solution seems a bit complicated to me. Just look at all of the possible substrings and check them individually:

def palindromes(text):
    text = text.lower()
    results = []

    for i in range(len(text)):
        for j in range(0, i):
            chunk = text[j:i + 1]

            if chunk == chunk[::-1]:
                results.append(chunk)

    return text.index(max(results, key=len)), results

text.index() will only find the first occurrence of the longest palindrome, so if you want the last, replace it with text.rindex().

The following function returns the longest palindrome contained in a given string. It is just slightly different in that it uses itertools as suggested in this answer. There is value in abstracting away the combination generation. Its time complexity is evidently still cubic. It can trivially be adapted as needed to return the index and/or the list of palindromes.

import itertools

def longest_palindrome(s):
    lp, lp_len = '', 0
    for start, stop in itertools.combinations(range(len(s)+1), 2):
        ss = s[start:stop]  # substring
        if (len(ss) > lp_len) and (ss == ss[::-1]):
            lp, lp_len = ss, len(ss)
    return lp

below is a code I wrote for the same question. It might not be really optimized but works like a charm. Pretty easy to understand too for beginners

def longestPalindrome(s):
        pal = []
        longestpalin = s
        l = list(s)
        if len(s)>0:
            if len(s)==2:
                p = l
                if p[0]==p[1]:
                    return s
                else:
                    return l[0]
            else:
                for i in range(0,len(l)):
                    for j in range(i+1,len(l)+1):
                        p = l[i:j]
                        if p == p[::-1]:
                            if len(p)>len(pal):
                                pal = p
                                p = ''.join(p)
                                longestpalin = p
            return longestpalin
        else:
            return longestpalin
Gaurav Padgilwar
a = "xabbaabba"  # Provide any string

count=[]
for j in range(len(a)):
    for i in range(j,len(a)):
        if a[j:i+1] == a[i:j-1:-1]:      
            count.append(i+1-j)

print("Maximum size of Palindrome within String is :", max(count))
Abhishek Mishra

If you like the recursive solution, I have written a recursive version. It is also intuitive.

def palindrome(s):
  if len(s) <= 1:
    return s
  elif s[0] != s[-1]:
    beginning_palindrome = palindrome(s[:-1])
    ending_palindrome = palindrome(s[1:])
    if len(beginning_palindrome) >= len(ending_palindrome):
      return beginning_palindrome
    else:
      return ending_palindrome
  else:
    middle_palindrome = palindrome(s[1:-1])
    if len(middle_palindrome) == len(s[1:-1]):
        return s[0] + middle_palindrome + s[-1]
    else:
        return middle_palindrome

Here's a code you can use for finding the longest palindromic substring:

string = "sensmamstsihbalabhismadamsihbala"
string_shortener = ""
pres = 0
succ = 3
p_temp=0
s_temp=0
longest = ""
for i in range(len(string)-2):
    string_shortener = string[pres:succ]
    if(string_shortener==string_shortener[::-1]):
       p_temp = pres
       s_temp = succ
       for u in range(1000):
           p_temp-=1
           s_temp +=1
           string_shortener = string[p_temp:s_temp]
           if(string_shortener == string_shortener[::-1]):
                if len(string_shortener)>len(longest):
                    longest = string_shortener
            else:
                break
    pres+=1
    succ+=1
print(longest)

inputStr = "madammmdd"
outStr = ""
uniqStr = "".join(set(inputStr))
flag = False
for key in uniqStr:
   val = inputStr.count(key)
   if val % 2 !=0:
      if not flag:
         outStr = outStr[:len(outStr)/2]+key+outStr[len(outStr)/2:]
         flag=True
      val-=1
   outStr=key*(val/2)+outStr+key*(val/2)
print outStr

I have made function name as maxpalindrome(s) in this one string argument 's'. This function will return longest possible palindrome sub string and length of substring...

def maxpalindrome(s):
if len(s) == 1 or s == '':
    return str(len(s)) + "\n" + s
else:
    if s == s[::-1]:
        return str(len(s)) + "\n" + s
    else:
        for i in range(len(s)-1, 0, -1):
            for j in range(len(s)-i+1):
                temp = s[j:j+i]
                if temp == temp[::-1]:
                    return str(len(temp)) +"\n"+temp

Here is another clean and simple approach taken from the excellent online course Design of Computer Programs by P. Norvig. It iterates over all characters in the string and attempts to "grow" the string to both left and right.

def longest_sub_palindrome_slice(text):
    "Return (i,j) such that text[i,j] is the longest palindrome in text"
    if text == '': return (0, 0)
    def length(slice): a,b = slice; return b-a
    candidates = [grow(text, start, end)
                 for start in range(len(text))
                 for end in (start, start + 1)]
    return max(candidates, key=length)

def grow(text, start, end):
    "Start with a 0- or 1- length palindrome; try to grow a bigger one"
    while (start > 0 and end < len(text)
           and text[start-1].upper() == text[end].upper()):
        start -= 1; end += 1
    return (start, end)

value ="Madamaaamadamaaaacdefgv"
longestPalindrome =""
lenght =0;
for i in range(len(value)):
        for j in range(0, i):
            array = value[j:i + 1]
            if (array == array[::-1] and len(longestPalindrome) < len(array)):
                longestPalindrome =array
print(longestPalindrome)


s='stresseddesserts'
out1=[]
def substring(x):
    for i in range(len(x)):
        a=x[i:]
        b=x[:-i]
        out1.append(a)
        out1.append(b)
        
    return out1

for i in range(len(s)):
    substring(s[i:])    
final=set([item for item in out1 if len(item)>2])
final
palind={item:len(item) for item in final if item==item[::-1]}
print(palind)
sorted(palind.items(),reverse=True, key=lambda x: x[1])[0]

{'tresseddessert': 14, 'seddes': 6, 'esseddesse': 10, 'esse': 4, 'stresseddesserts': 16, 'resseddesser': 12, 'edde': 4, 'sseddess': 8}

('stresseddesserts', 16)

Muskaan
def longestPalindrome(s):
        temp = ""
        for i in range(len(s)):
            for j in range(len(s)-1,i-1,-1):
                if s[i] == s[j]:
                    m = s[i:j+1]
                    if m == m[::-1]:
                        if len(temp) <= len(m):
                            temp = m
        return temp

I have to agree the solution may seem to complicated, i think the best solution, to find the largest palindrome in a subsequence, (considering characters in between for example in 'character' the largest palindrome should be carac) is:

def find_char_backwards(a, c):
for i in range(len(a) - 1, -1,-1):
    if a[i] == c:
        index=i
        return True, index

return False, 0

def longest_palindorme(a):
if len(a) < 2:
    return a
else:
    c=a[0]
    (exist_char,index) = find_char_backwards(a[1:],c)
    if exist_char:
        palindrome=[c] + longest_palindorme(a[1:index+1]) + [c]
    else:
        palindrome=[]
    rest_palidorme=longest_palindorme(a[1:])

if len(palindrome)>len(rest_palidorme):
    return palindrome
else:
    return rest_palidorme

Where a is an array, this solution uses recursion, and dynamic programming

Bob

Use a nested loop:

for x in range(len(body)):
    for y in range(len(body)):
    ...
标签
python
palindrome
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