add level 1 problems

Author: felixfirefighter

deleteAtTail.java

@@ -0,0 +1,15 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public ListNode deleteAtTail(ListNode head) {
+  if(head == null || head.next == null) return null;
+  
+  ListNode node = head;
+  
+  while(node.next.next != null) {
+      node = node.next;
+  }
+
+  node.next = null;
+  return head;
+}

findMiddleNode.java

@@ -0,0 +1,15 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public ListNode findMiddleNode(ListNode head) {
+  if(head == null || head.next == null) return head;
+
+  ListNode fast = head;
+  ListNode slow = head;
+  while(fast.next != null && fast.next.next != null) {
+      slow = slow.next;
+      fast = fast.next.next;
+  }
+
+  return slow;
+}

flipHorizontalAxis.java

@@ -0,0 +1,14 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public static void flipHorizontalAxis(int[][] matrix) {
+  int rowLength = matrix.length - 1;
+  
+  for(int i = 0; i < matrix.length / 2; i++) {
+      for(int j = 0; j < matrix[i].length; j++) {
+          int temp = matrix[i][j];
+          matrix[i][j] = matrix[rowLength - i][j];
+          matrix[rowLength - i][j] = temp;
+      } 
+  }
+}

isStringPalindrome.java

@@ -0,0 +1,19 @@
+// java.util.* and java.util.streams.* have been imported for this problem.
+// You don't need any other imports.
+
+public static boolean isStringPalindrome(String str){
+    if(str == null || str.length() == 0) return true;
+    
+    int lastIndex = str.length() - 1;
+    
+    for(int i = 0; i <= lastIndex / 2; i++) {
+        
+        char start = str.charAt(i);
+        char end = str.charAt(lastIndex - i);
+        if(start != end) {
+            return false;
+        }
+    }
+    
+    return true;
+}