packageeasy;

importjava.util.HashMap;

importjava.util.Map;

publicclassIsomorphicStrings {

publicbooleanisIsomorphic(Strings,Stringt) {

if(s ==null ||s.length() ==0)return (t ==null ||t.length() ==0);

if(t ==null ||t.length() ==0)return (s ==null ||s.length() ==0);

char[]schar =s.toCharArray();

char[]tchar =t.toCharArray();

Map<Character,Character>map =newHashMap();

if(s.length() !=t.length())returnfalse;

for(inti =0;i <s.length();i++){

if(map.containsKey(schar[i])){

if(map.get(schar[i]) !=tchar[i])returnfalse;

}else {

if(map.containsValue(tchar[i]))returnfalse;//this line is necessary for this case: ("ab", "aa")

map.put(schar[i],tchar[i]);

}

}

returntrue;

}

}

|257|[Binary Tree Paths](https://leetcode.com/problems/binary-tree-paths/)|[Solution](../../blob/master/EASY/src/easy/BinaryTreePaths.java) | O(n*h) | O(h) | DFS/Recursion

|209|[Minimum Size Subarray Sum](https://leetcode.com/problems/minimum-size-subarray-sum/)|[Solution](../../blob/master/MEDIUM/src/medium/MinimumSizeSubarraySum.java)| O(n)|O(1)| Medium|

|206|[Reverse Linked List](https://leetcode.com/problems/reverse-linked-list/)|[Solution](../../blob/master/EASY/src/easy/ReverseLinkedList.java)| O(n)|O(1) | Easy

|205|[Isomorphic Strings](https://leetcode.com/problems/isomorphic-strings/)|[Solution](../../blob/master/EASY/src/easy/IsomorphicStrings.java)| O(n)|O(1) | Easy

|200|[Number of Islands](https://leetcode.com/problems/number-of-islands/)|[Union Find](../../blob/master/MEDIUM/src/medium/NumberOfIslandsUnionFind.java)[DFS](../../blob/master/MEDIUM/src/medium/NumberofIslandsDFS.java)| O(m*n)|O(m*n) | Medium| Union Find, DFS

|189|[Rotate Array](https://leetcode.com/problems/rotate-array/)|[Solution](../../blob/master/EASY/src/easy/RotateArray.java)| O(n)|O(n), could be optimized to O(1) | Easy

|173|[Binary Search Tree Iterator](https://leetcode.com/problems/binary-search-tree-iterator/)|[Queue](../../blob/master/MEDIUM/src/medium/BSTIterator_using_q.java)[Stack](../../blob/master/MEDIUM/src/medium/BSTIterator_using_stack.java)| O(1)|O(h)| Medium|