[leetcode] 647. Palindromic Substrings @ python

原题

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.

Example 1:
Input: “abc”
Output: 3
Explanation: Three palindromic strings: “a”, “b”, “c”.
Example 2:
Input: “aaa”
Output: 6
Explanation: Six palindromic strings: “a”, “a”, “a”, “aa”, “aa”, “aaa”.
Note:
The input string length won’t exceed 1000.

解法

遍历法, 从每个index开始, 遍历从它开始的所有substring, 检查是否为回文.
Time: O(n*n/2)
Space: O(1)

代码

class Solution(object):
    def countSubstrings(self, s):
        """
        :type s: str
        :rtype: int
        """
        ans = 0
        for start in range(len(s)):
            for end in range(start,len(s)):
                win = s[start: end +1]
                if win == win[::-1]:
                    ans += 1
                    
        return ans