647_Palindromic Substrings
647. Palindromic Substrings
Level: medium
Tag: string, dynamic programming
Question
Given a string, your task is to count how many palindromic substrings in this string.
The substrings with different start indexes or end indexes are counted as different substrings
even they consist of same characters.
Note: A palindrome is a word, which reads the same backward as forward.
Example1
Input: "abc"
Output: 3
Explanation: Three palindromic strings: "a", "b", "c".
Example2
Input: "aaa"
Output: 6
Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".
Idea: (expand around center, dynamic programming, similar to Longest Palindromic Substring Q5)
Loop from the beginning to the end. Take each letter be the center of the substring, and find the number of palindromic substring centered by that letter.
There are two types of center:
the letter is the center, such that the palindromic substring has odd number of letters
the space on the right of the letter is the center, such that the palindromic substring has even number of letters
Complexity
Time:
Space:
Solution
class Solution(object):
def countSubstrings(self, s):
"""
:type s: str
:rtype: int
"""
count = 0
# dynamic programming
for i in range(len(s)):
count += self.helper(s,i,i+1) + self.helper(s,i,i)
return count
# helper function to get the number of palindromic substring expanding from s[l:r+1]
# from center to outer letters
def helper(self, s, l, r):
"""
:type s: str
:type l: num
:type r: num
:rtype: str
"""
count = 0
while l >= 0 and r <= len(s)-1 and s[l] == s[r]:
count += 1
l -= 1
r += 1
return count
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