add 3 problems

Author: felixfirefighter

deleteAtMiddle.java

@@ -0,0 +1,23 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public ListNode deleteAtMiddle(ListNode head, int position) {
+  if(head == null) return head;
+  
+  // if head needs to be deleted
+  if(position == 1) { 
+      return head.next;
+  }
+  
+  ListNode prev = head;
+  
+  // find prev node of the node to be deleted
+  for(int i=1;i<position-1;i++) {
+      if(prev == null || prev.next == null) return head;
+      prev = prev.next;
+  }
+  
+  prev.next = prev.next.next;
+  
+  return head;
+}

insertAtHead.java

@@ -0,0 +1,11 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public ListNode insertAtHead(ListNode head, int data) {
+  ListNode node = new ListNode(data);
+  
+  if(head == null) return node;
+          
+  node.next = head;
+  return node;
+}

isIsomorphic.java

@@ -0,0 +1,25 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public static boolean isIsomorphic(String input1, String input2) {
+  if((input1 == null || input2 == null)) return true;
+  if(input1.length() != input2.length()) return false;
+  
+  Hashtable<Character, Integer> table1 = new Hashtable<Character, Integer>();
+  Hashtable<Character, Integer> table2 = new Hashtable<Character, Integer>();
+  
+  for(int i=0;i<input1.length();i++) {
+      char c1 = input1.charAt(i);
+      char c2 = input2.charAt(i);
+      
+      int count1 = table1.getOrDefault(c1, 0) + 1;
+      table1.put(c1, count1);
+      
+      int count2 = table2.getOrDefault(c2, 0) + 1;
+      table2.put(c2, count2);
+      
+      if(count1 != count2) return false;
+  }
+  
+  return true;
+}