leetcode 647 palindromic substrings

leetcode 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".

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solution one

expand around center

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

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solution two

dp

class Solution {
    public int countSubstrings(String s) {
        int res = 0;
        int n = s.length();
        boolean dp[][] = new boolean[n][n];
        for (int i = 0; i < n; i++) {
            for ( int j = i; j >= 0; j--) {
                dp[i][j] = (s.charAt(i) == s.charAt(j)) && (i-j < 3 || dp[i-1][j+1]);
                if (dp[i][j]){
                    res++;
                }
            }
        }
        return res;
    }
}