647. Palindromic Substrings

• Difficulty: Medium

• Topics: String, Dynamic Programming

• Similar Questions:

  • Longest Palindromic Substring

  • Longest Palindromic Subsequence

  • Palindromic Substrings

Problem:

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.

Solutions:

class Solution {
public:
    int countSubstrings(string s) {
        int sum = 0;
        for (int i = 0; i < s.length(); ++i) {
            sum += palindromeNum(s, i, i);
            sum += palindromeNum(s, i, i + 1);
        }

        return sum;
    }

private:
    int palindromeNum(const string& s, int left, int right) {
        int count = 0;
        while (left >= 0 && right < s.length()) {
            if (s[left] == s[right]) {
                ++count;
                --left;
                ++right;
            } else {
                return count;
            }
        }

        return count;
    }

};