[Leetcode]07/08 [hash] Isomorphic Strings

/* Method1: Time:O(n^2); Space:O(n) Logic: check each correpsonding character set if in the map and backward check if the corresponding key of c2 is the same as current c1 getKey() Method2: Time:O(n):Space:O(2n) Logic: Two hashmap sourceMap[ t[x] ] = s[x] targetMap[ s[x] ] = t[x] False if two map relationship valid;Otherwise, it is ture. Given "egg", "add", return true. Given "foo", "bar", return false. Given "paper", "title", return true. */

public class Solution {
    public boolean isIsomorphic(String s, String t) {
        
        
        /* Metho1: 
        // special case
        if (  s== null || t == null )
        {
            return false;
        }
        if ( s.length() != t.length() )
        {
            return false;
        }
        if (  s.length() ==0 && t.length() == 0 )
        {
            return true;
        }
        
        HashMap map = new HashMap();
        for ( int i = 0; i < s.length();i++  )
        {
            char c1 = s.charAt(i);
            char c2 = t.charAt(i);
            
            //Input:
            //  "ab", "aa"
            //Output:
            //true
            //Expected:
            //false
            // **** Need to backward check
            
            //if ( map.containsKey(c1) )
            //{
            //    if ( c2 != map.get(c1) )
            //    {
            //        return false;
            //   }
            //}
            //else 
            //{
            //    map.put(c1,c2);
            //}
            
            
            Character c = getKey(map,c2);
            if ( c != null && c != c1)
            {
                return false;
            }
            else if ( map.containsKey(c1) )
            {
                if ( c2 != map.get(c1) )
                {
                    return false;
                }
            }
            else 
            {
                map.put(c1,c2);
            }
            
        }
        return true;
        */
        
        
        
        
        /*Method2:*/
        if ( s.length() != t.length() )
        {
            return false;
        }
        if ( s.length() == 0 && t.length() == 0)
        {
            return true;
        }
        if ( s == null || t == null )
        {
            return false;
        }
        HashMap map1 = new HashMap();
        HashMap map2 = new HashMap();
        for (  int i = 0 ;i < s.length() ;i ++)
        {
            char c1 = s.charAt(i);
            char c2 = t.charAt(i);
            if (  map1.containsKey(c1) )
            {
                if ( map1.get(c1) != c2 ) 
                {
                    return false;
                }
            }
            if ( map2.containsKey(c2) )
            {
                if ( map2.get(c2) != c1 )
                {
                    return false;
                }
            }
            map1.put( c1,c2 );
            map2.put( c2,c1);
        }
        return true;
    }
    
    
    // a method for getting key given a target value
    // a method for getting key of a target value
    
    private Character getKey(HashMap map, Character value )
    {
        for ( Map.Entry entry : map.entrySet() )
        {
            if ( entry.getValue().equals(value) )
            {
                return entry.getKey();
            }
        }
        return null;
    }
    
}