647. Palindromic Substrings

Given a string s, return the number of palindromic substrings in it.

A string is a palindrome when it reads the same backward as forward.

A substring is a contiguous sequence of characters within the string.

 

Example 1:

Input: s = "abc"
Output: 3
Explanation: Three palindromic strings: "a", "b", "c".

Example 2:

Input: s = "aaa"
Output: 6
Explanation: Six palindromic strings: "a", "a", "a", "aa", "aa", "aaa".

 

Constraints:

• 1 <= s.length <= 1000
• s consists of lowercase English letters.

class Solution {
    public int countSubstrings(String s) {
        int n = s.length();
        int res = n;
        for(int i = 2; i <= n; i++) {
            for(int j = 0; j <= n - i; j++) {
                if(help(s.substring(j, j + i))) {
                    res++;
                }
            }
        }
        return res;
    }
    public boolean help(String s) {
        int l = 0, r = s.length() - 1;
        while(l < r) {
            if(s.charAt(l) != s.charAt(r)) return false;
            l++;
            r--;
        }
        return true;
    }
}

mlgb, brute force管上。

class Solution {
    int res = 0;
    public int countSubstrings(String s) {
        int n = s.length();
        for(int i = 0; i < s.length(); i++) {
            help(s, i, i);
            help(s, i, i + 1);
        }
        return res;
    }
    
    public void help(String s, int l, int r) {
        while(l >= 0 && r <= s.length() - 1 && s.charAt(l) == s.charAt(r)) {
            res++;
            l--;
            r++;
        }
    }
}

或者用一种和5一样的midexpansion方法，每次check这一位开始的odd和even的substring看是不是palindrome。