647. Palindromic Substrings

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:

1. The input string length won't exceed 1000.

 

Approach #1: Greedy. [Java]

class Solution {
    int count = 0;
    public int countSubstrings(String s) {
        if (s == null || s.length() == 0) return 0;
        for (int i = 0; i < s.length(); ++i) {
            solve(s, i, i);
            solve(s, i, i+1);
        }
        return count;
    }
    
    public void solve(String s, int left, int right) {
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            left--;
            right++;
            count++;
        }
    }
}

　　

Analysis:

Step1: Start a for loop to point at every single character from where we will trace the palindrome string.

solve(s, i, i);　　// To check the palindrome of odd length palindromic subsring.

solve(s, i, i+1); 　　// To check the palindrome of even length palindrome substring.

 

Step2: From each character of the string, we will keep checking is the substring is a palindrome and increment the palindrome count. To check the palindrome, keep checking the left and right of the character is it is same or not.

 

Reference:

https://leetcode.com/problems/palindromic-substrings/discuss/105688/Very-Simple-Java-Solution-with-Detail-Explanation