Leetcode: Palindromic Substrings

647. Palindromic Substrings
Medium

1656

85

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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.
 

 

DP:

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

Extend palindrome: (better)

public 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++) { // i is the mid point
            extendPalindrome(s, i, i); // odd length;
            extendPalindrome(s, i, i + 1); // even length
        }
        
        return count;
    }
    
    private void extendPalindrome(String s, int left, int right) {
        while (left >=0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
            count++; left--; right++;
        }
    }
}