[Leetcode 5 ]can't really find what's wrong in my solution for longest palindrome substring problem

Question

1.00/5 (2 votes)

See more:

input:
babad
abbd

output:
ad
bb

expected:
bab
bb

C++

#include<iostream>
using namespace std;
class Solution {
public:
        string longestPalindrome(string s) {
        int maxlength=1;
        bool ispalindromic[1000][1000]={false};
        for(int i=0;i<s.length();i++)
                ispalindromic[i][i]=1;
                
        for(int l=2;l<s.length();l++){
        for(int i=0;i<s.length()-1; i++){
            int j=i+l-1;
            if(l==2&&s[i]==s[j]){
                ispalindromic[i][j]=1;
            maxlength=max(maxlength,j-i+1);
                continue;}
            if(ispalindromic[i+1][j-1]&&s[i]==s[j]){
                ispalindromic[i][j]=1;
            maxlength=max(maxlength,j-i+1);
            }
        }}
        for(int i=0;i<s.length();i++){
        int j=i+maxlength-1;
            if(ispalindromic[i][j]){
                   return s.substr(i,j);
                }
            }
        return s.substr(0,1);
        }
};

What I have tried:

I created ispalindromic[1000][1000] first and made sure that every alphabet itself is palindromic. Then I check palindromic from the length of 2 and so on. Whenever ispalindromic becomes true, the code updates maxlength so that in the end the code can simply use maxlength to print longest palindromic.

I also added cout<<j<<endl; to see whether it's j's fault. if i input abbd, j is 2 which is right, and program prints right answer bb, but if I input acddc, j is still right(3), but the program prints ddc which is wrong.
 
if(ispalindromic[i][j]){
            	   cout<<j<<endl;
                   return s.substr(i,j);
                }

Posted 4-Aug-20 20:11pm

JacksonSteel

Updated 6-Aug-20 16:45pm

v5

Add a Solution

2 solutions

Add a Solution

Add your solution here

Treat my content as plain text, not as HTML

Preview 0

…

Existing Members

Sign in to your account

...or Join us

Download, Vote, Comment, Publish.

Your Email
Password
Forgot your password?

Your Email
This email is in use. Do you need your password?
Optional Password

I have read and agree to the Terms of Service and Privacy Policy
Please subscribe me to the CodeProject newsletters

When answering a question please:

Read the question carefully.
Understand that English isn't everyone's first language so be lenient of bad spelling and grammar.
If a question is poorly phrased then either ask for clarification, ignore it, or edit the question and fix the problem. Insults are not welcome.
Don't tell someone to read the manual. Chances are they have and don't get it. Provide an answer or move on to the next question.

Let's work to help developers, not make them feel stupid.

This content, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

OriginalGriff · Answer 1 · 2020-08-04T20:22:00

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:

Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16

Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:

C#

int Double(int value)
   {
   return value * value;
   }

Fortunately, you have a tool available to you which will help you find out what is going on: the debugger. How you use it depends on your compiler system, but a quick Google for the name of your IDE and "debugger" should give you the info you need.

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?

Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!

Patrice T · Answer 2 · 2020-08-04T23:58:00

Advice: Learn to indent properly your code, it show its structure and it helps reading and understanding. It also helps spotting structures mistakes.

C++

#include<iostream>
using namespace std;
class Solution {
    public:
    string longestPalindrome(string s) {
        int maxlength=1;
        bool ispalindromic[1000][1000]={false};
        for(int i=0;i<s.length();i++)
            ispalindromic[i][i]=1;

        for(int l=2;l<s.length();l++){
            for(int i=0;i<s.length()-1; i++){
                int j=i+l-1;
                if(l==2&&s[i]==s[j]){
                    ispalindromic[i][j]=1;
                    maxlength=max(maxlength,j-i+1);
                continue;}
                if(ispalindromic[i+1][j-1]&&s[i]==s[j]){
                    ispalindromic[i][j]=1;
                    maxlength=max(maxlength,j-i+1);
                }
            }}
            for(int i=0;i<s.length();i++){
                int j=i+maxlength-1;
                if(ispalindromic[i][j]){
                    return s.substr(i,j);
                }
            }
            return s.substr(0,1);
        }
    }

Indentation style - Wikipedia[^]

Professional programmer's editors have this feature and others ones such as parenthesis matching and syntax highlighting.
Notepad++ Home[^]
ultraedit[^]
-----
In indented code:

C++

#include<iostream>
using namespace std;
class Solution {
    public:
    string longestPalindrome(string s) {
        int maxlength=1;
        bool ispalindromic[1000][1000]={false};
        for(int i=0;i<s.length();i++)
            ispalindromic[i][i]=1;

        for(int l=2;l<s.length();l++){
            for(int i=0;i<s.length()-1; i++){
                int j=i+l-1;
                if(l==2&&s[i]==s[j]){
                    ispalindromic[i][j]=1;
                    maxlength=max(maxlength,j-i+1);
                continue;}
                if(ispalindromic[i+1][j-1]&&s[i]==s[j]){
                    ispalindromic[i][j]=1;
                    maxlength=max(maxlength,j-i+1);
                }
            }} // the double } here is saving 1 keystroke at cost of obfuscationg the code strubture
            for(int i=0;i<s.length();i++){
                int j=i+maxlength-1;
                if(ispalindromic[i][j]){
                    return s.substr(i,j);
                }
            }
            return s.substr(0,1); // This return is inside a for loop, surprising !
            // it prevent the loop from looping !
        }
    }

In your code, I see a maxlength for the length of palindrome, but I don't see a maxposition for the position of palindrome.
-----
Your code do not behave the way you expect, or you don't understand why !

There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.

Debugger - Wikipedia, the free encyclopedia[^]

Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[^]
Basic Debugging with Visual Studio 2010 - YouTube[^]

1.11 — Debugging your program (stepping and breakpoints) | Learn C++[^]

The debugger is here to only show you what your code is doing and your task is to compare with what it should do.

[Update]
Your code is way complicated.
Try by hand to solve the problem and see if you need a huge Boolean array to get the answer.

[Leetcode 5 ]can't really find what's wrong in my solution for longest palindrome substring problem

2 solutions

Solution 1

Solution 2

Add your solution here

Preview 0