# Java solution using HashMap

• public class Solution {
public boolean isIsomorphic(String s, String t) {
if(s == null || s.length() <= 1) return true;
HashMap<Character, Character> map = new HashMap<Character, Character>();
for(int i = 0 ; i< s.length(); i++){
char a = s.charAt(i);
char b = t.charAt(i);
if(map.containsKey(a)){
if(map.get(a).equals(b))
continue;
else
return false;
}else{
if(!map.containsValue(b))
map.put(a,b);
else return false;

}
}
return true;

}
}

• No,this solution is O(N*N), because of the containsValue() method! Every search of containsValue() is O(N).

• Hi
can you tell me what is wrong in my code? I tried to write it in C++, but mine is wrong, thanks

class Solution {
public:
bool isIsomorphic(string s, string t) {
unordered_map<char,char>table;
for(int i=0; i<s.size(); ++i){
if(table.count(s[i])){
if(table[s[i]]!=t[i]) return false;
}
else{
if(!table.count(t[i])) table[s[i]]=t[i];
else                   return false;
}
}
return true;
}
};

• the value could be stored in a HashSet object so the time efficiency becomes O(n)

• It's O(1) for containsValue() in hashmap

• I think the containValue() is O(n)
http://stackoverflow.com/questions/16757359/what-is-the-time-complexity-of-hashmap-containsvalue-in-java
you need to iterate through the (key,value) pair to find the value you want, that's why it's O(n).

• I first used map.get(a) != b instead of !map.get(a).equals(b), and it's not working. Could anyone clarify why it happens?

• @mengmengli815 map.get(a) return object of type Character. you might want to cast it to char before comparing.

• This post is deleted!

• public boolean isIsomorphic(String s, String t) {
if (s == null || t == null) return false;
if (s.length() != t.length()) return false;

Map<Character, Integer> mapS = new HashMap<Character, Integer>();
Map<Character, Integer> mapT = new HashMap<Character, Integer>();

for (int i = 0; i < s.length(); i++) {
int indexS = mapS.getOrDefault(s.charAt(i), -1);
int indexT = mapT.getOrDefault(t.charAt(i), -1);

if (indexS != indexT) {
return false;
}

mapS.put(s.charAt(i), i);
mapT.put(t.charAt(i), i);
}

return true;
}

Two strings are isomorphic if the positions of the characters follow the same pattern. So I'm using maps to compare the position patterns.

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