[LC] 647. Palindromic Substrings

Posted 2021-03-03 xuanlu

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".

 

Solution 1

class Solution {
    public int countSubstrings(String s) {
        int len = s.length();
        int res = 0;
        boolean [][] isPalin = new boolean[len][len];
        for (int i = 0; i < len; i++) {
            for (int j = 0; j <= i; j++) {
                if (s.charAt(i) == s.charAt(j) && (i <= j + 2 || isPalin[i - 1][j + 1])) {
                    isPalin[i][j] = true;
                    res += 1;
                }
            }
        }
        return res;
    }
}

 

Solution 2

class Solution {
    public int countSubstrings(String s) {
        int len = s.length();
        int[] count = new int[1];
        for (int i = 0; i < len; i++) {
            getPalin(s, i, i, count);
            getPalin(s, i, i + 1, count);
        }
        return count[0];
    }
    
    private void getPalin(String s, int left, int right, int[] count) {
        // need to ensure left, right value for while
        while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            count[0] += 1;
            left -= 1;
            right += 1;
        }
    } 
}