I have solved the GROUP ANAGRAM question in leetcode in two ways.Why one of them is not giving correct result whereas the other one is returing null?

Given an array of strings strs, group the anagrams together. You can return the answer in any order.

Solution 1:the correct one:

public List<List<String>> groupAnagrams(String[] strs) {
        HashMap<String,List<String>> hm=new HashMap<>();   
        
        for(String st:strs){
            char[] arr=st.toCharArray();
            Arrays.sort(arr);
            String cannonical=new String(arr);
            if(!hm.containsKey(cannonical)){
                hm.put(cannonical,new LinkedList<String>());
            }
            
            hm.get(cannonical).add(st);
        }
       
        return new LinkedList<>(hm.values());
    }

Solution 2:Incorrect :

public List<List<String>> groupAnagrams(String[] strs) {
        HashMap<String,List<String>> hm=new HashMap<>();   
        
        for(String st:strs){
            char[] arr=st.toCharArray();
            Arrays.sort(arr);
            String cannonical=new String(arr);
            
             hm.getOrDefault(cannonical,new LinkedList<String>()).add(st);
            
        }
        return new LinkedList<>(hm.values());
    }

why soln 2 is not working when both the codes are very similar

In Solution 1, when you create a new entry in the HashMap using hm.put(cannonical,new LinkedList<String>()) , you ensure that if the cannonical key doesn't exist in the HashMap, a new LinkedList is created and associated with that key.

This way, when you later call hm.get(cannonical).add(st), you are sure that there is a List<String> associated with the cannonical key, and you can safely add the current string st to that list.

In Solution 2, the incorrect part is here:

hm.getOrDefault(cannonical, new LinkedList<String>()).add(st);

The getOrDefault method will return either the existing list associated with the cannonical key or the newly created list, but this newly created list is never actually put into the HashMap. So, even though you correctly retrieve the list, you're modifying a list that's not associated with any key in the HashMap.

The correct version would be:

hm.putIfAbsent(cannonical, new LinkedList<String>());
hm.get(cannonical).add(st);

Here, putIfAbsent ensures that a list is associated with the cannonical key, and then you can safely add the current string to that list.

This is the problem.

 hm.getOrDefault(cannonical,new LinkedList<String>()).add(st);

There is no key so you are trying to add a value to a list that isn't mapped.

Replace it with

hm.computeIfAbsent(cannonical, v-> new LinkedList<>()).add(st);

The above says, if cannonical is not present, create a value for it, and return that value, in this case a list. Then you can add the string to the list.
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