Max length of substring-palindrome

[member 428835]

Hi PF!

I'm trying to find the max length of a sub-string palindrome. My logic is to have a left-most index called start that starts from string loc 0 and traverses to the end of the string via a for loop. Next I was thinking a while loop as long as L < R: I'd have a left index L=start and a right index R = string.length(). If $L \neq R$, then $R = R-1$ and continue the while loop. However, if $L = R$ then $L++$ and $R--$, we count this as a match, and continue. Only until L and R are the same (odd palindrome) or 1 away (even palindrome) do we replace the max palindrome length. However, I have a few bugs. Here's what I have, and any help is much appreciated.

#include <iostream>
#include <vector>
#include <string> // for string class
class Solution {
public:
    int longestPalindrome(std::string str) {
        int n = str.length();
        int maxLen = 1;
        int count = 0;
        for (int start = 0; start < n; start++) {
            int L = start;
            int R = n - 1;
           
            while (R > L) {
                if (str[R] != str[L]) {
                    R--;
                    continue;
                }
                count ++;
                if (L == R) {
                    maxLen = std::max(maxLen, 2*count+1);
                    count = 0;
                    break;
                }
                if (L == R - 1) {
                    maxLen = std::max(maxLen, 2*count);
                    count = 0;
                    break;
                }
                L++;
                R--;
            }
        }
        return maxLen;
    }
};
int main()
{
    std::string s_ = "yyureeeerl";
    Solution ob1;
    auto sol = ob1.longestPalindrome(s_);
    std::cout << sol << "\n";

    return 0;
}

 

[ShayanJ]

I didn't spend that much time on your code, but the one obvious problem I can see is that the if(L==R) block is unreachable because the while loop is over when that happens!