Added codes for 21 March

GeeksForGeeks/March/21-3-24/GFG.java

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+//{ Driver Code Starts
+//Initial Template for Java
+
+import java.util.*;
+import java.util.HashMap;
+import java.io.*;
+
+    class Node
+    {
+        int data;
+        Node left,right;
+        Node(int data){
+        this.data = data;
+        left=null;
+        right=null;
+    }
+    }
+
+public class GFG2
+{
+    static Node buildTree(String str){
+
+        if(str.length()==0 || str.charAt(0)=='N'){
+            return null;
+        }
+
+        String ip[] = str.split(" ");
+        // Create the root of the tree
+        Node root = new Node(Integer.parseInt(ip[0]));
+        // Push the root to the queue
+
+        Queue<Node> queue = new LinkedList<>(); 
+
+        queue.add(root);
+        // Starting from the second element
+
+        int i = 1;
+        while(queue.size()>0 && i < ip.length) {
+
+            // Get and remove the front of the queue
+            Node currNode = queue.peek();
+            queue.remove();
+
+            // Get the current node's value from the string
+            String currVal = ip[i];
+
+            // If the left child is not null
+            if(!currVal.equals("N")) {
+
+                // Create the left child for the current node
+                currNode.left = new Node(Integer.parseInt(currVal));
+                // Push it to the queue
+                queue.add(currNode.left);
+            }
+
+            // For the right child
+            i++;
+            if(i >= ip.length)
+                break;
+
+            currVal = ip[i];
+
+            // If the right child is not null
+            if(!currVal.equals("N")) {
+
+                // Create the right child for the current node
+                currNode.right = new Node(Integer.parseInt(currVal));
+
+                // Push it to the queue
+                queue.add(currNode.right);
+            }
+            i++;
+        }
+
+        return root;
+    }
+
+	public static void inorder(Node root)
+	{
+	    if(root==null)
+	    return;
+	    inorder(root.left);
+	    System.out.print(root.data);
+	    inorder(root.right);
+	}
+     /* Drier program to test above functions */
+    public static void main(String args[]) throws IOException
+    {
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+
+	        int t=Integer.parseInt(br.readLine());
+
+	        while(t > 0){
+	            String s = br.readLine();
+    	    	Node root = buildTree(s);
+        	    GFG g = new GFG();
+
+			    ArrayList<Integer> res = g.zigZagTraversal(root) ;
+			    for (int i = 0; i < res.size (); i++)
+			        System.out.print (res.get (i) + " ");
+			    System. out. println();  
+
+                t--;
+
+        }
+	}
+}
+// } Driver Code Ends
+
+
+//User function Template for Java
+
+/*class Node
+{
+    int data;
+    Node left,right;
+    Node(int d)
+    {
+        data=d;
+        left=right=null;
+    }
+}*/
+
+class GFG
+{
+    //Function to store the zig zag order traversal of tree in a list.
+	ArrayList<Integer> zigZagTraversal(Node root)
+	{
+	    //Add your code here.
+	    ArrayList<Integer> al = new ArrayList<>();
+        boolean flag = false;
+        Queue<Node> q = new LinkedList<>();
+        q.add(root);
+
+        while(!q.isEmpty())
+        {
+            int s = q.size();
+            ArrayList<Integer> temp = new ArrayList<>();
+
+            while(s>0)
+            {
+                Node n = q.remove();
+                s--;
+                if(flag)
+                    temp.add(0, n.data);
+                else
+                    temp.add(n.data);
+                if(n.left!=null)
+                    q.add(n.left);
+                if(n.right!=null)
+                    q.add(n.right);
+            }
+
+            flag = !flag;
+            al.addAll(temp);
+        }
+
+        return al;
+	}
+}

GeeksForGeeks/March/21-3-24/README.md

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+Time complexity - O(n)
+Space complexity - O(n)

LeetCode/March/21-3-24/README.md

@@ -0,0 +1,2 @@
+Time complexity - O(n)
+Space complexity - O(1)

LeetCode/March/21-3-24/Solution.java

@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution 
+{
+    public ListNode reverseList(ListNode head) 
+    {
+        ListNode prev = null;
+
+        while (head != null) 
+        {
+            ListNode next = head.next;
+            head.next = prev;
+            prev = head;
+            head = next;
+        }
+
+        return prev;    
+    }
+}