chore(java): left view of a tree (#305)

* Clone linked list with random pointer

* addition in  reviewers list

* Queues in Java

* Allocate Min Pages

* Left view of tree

* typo fixed

* Added more comments

* typo fixed

* indentation fixed

algorithms/Java/README.md

@@ -68,3 +68,4 @@
 ## Trees
 
 1. [Pre in Post Traversal](trees/pre-in-post-traversal.java)
+2. [Left View of a Tree](trees/left-view.java)

algorithms/Java/trees/left-view.java

@@ -0,0 +1,191 @@
+/* Given a Binary Tree, print Left view of it. 
+Left view of a Binary Tree is set of nodes visible when tree is visited from Left side. */
+import java.util.LinkedList; 
+import java.util.Queue; 
+import java.io.*;
+import java.util.*;
+
+class Node{
+    int data;
+    Node left;
+    Node right;
+    Node(int data){
+        this.data = data;
+        left=null;
+        right=null;
+    }
+}
+//Main Class
+class Main {
+    
+    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;
+    }
+    static void printInorder(Node root) {
+        //Print inorder of tree.
+        if(root == null)
+            return;
+            
+        printInorder(root.left);
+        System.out.print(root.data+" ");
+        
+        printInorder(root.right);
+    }
+    
+	public static void main (String[] args) throws IOException{
+        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+        //number of testcases.
+        int t=Integer.parseInt(br.readLine());
+
+        while(t > 0){
+            //Input a string for tree
+            String s = br.readLine();
+            Node root = buildTree(s);
+            Tree g = new Tree();
+            //calculate in left view of tree by calling function
+            ArrayList<Integer> result1 = g.leftViewIterative(root);
+            ArrayList<Integer> result2 = g.leftViewRecursive(root);
+            //print the result
+            System.out.println("Iterative Solution:");
+            for(int value : result1){
+                System.out.print(value + " ");
+            }
+            System.out.println("Recursive Solution:");
+            for(int value : result2){
+                System.out.print(value + " ");
+            }
+            System.out.println();
+            t--;
+        }
+    }
+}
+
+class Tree {
+    ArrayList<Integer> al;
+    //Iterative Approach
+    ArrayList<Integer> leftViewIterative(Node root) {
+        al = new ArrayList<Integer>();
+        if(root==null) return al;
+        //create a queue to store the node of treees
+        Queue<Node> q = new LinkedList<>();
+        q.add(root);
+        while(!q.isEmpty()){
+            int n = q.size();
+            //loop through available nodes in queue.
+            for(int i=1;i<=n;i++){
+                Node temp = q.poll();
+                //add first node from queue as it would be left most node
+                if(i==1) al.add(temp.data);
+                if(temp.left!=null) q.add(temp.left);
+                if(temp.right!=null) q.add(temp.right);
+            }
+        }
+        return al;
+    }
+
+    //Recursive Approach
+    int maxlvl=0;
+    ArrayList<Integer> leftViewRecursive(Node root) {
+        al = new ArrayList<Integer>();
+        //start from root of the tree
+        leftViewUtil(root, 1);
+        return al;
+    }
+    void leftViewUtil(Node node, int lvl){
+        if(node==null) return;
+        //first occurrence of the level will get added
+        if(maxlvl<lvl){
+            al.add(node.data);
+            //make the max level to curr level
+            maxlvl = lvl;
+        }
+        //increment the level of tree to get left most of next level
+        leftViewUtil(node.left, lvl+1);
+        leftViewUtil(node.right, lvl+1);
+    }
+}
+
+/* Input : 
+                 1
+               /   \
+              2     3
+             / \     \
+            4   5     6             
+Output : 
+Iterative Approach:
+1 2 4
+Recursive Approach:
+1 2 4
+
+Input :
+        1
+      /   \
+    2       3
+      \   
+        4  
+          \
+            5
+             \
+               6
+Output :
+Iterative Approach:
+1 2 4 5 6
+Recursive Approach:
+1 2 4 5 6
+
+Time Complexity: O(n) , where n is the number of nodes in the binary tree.
+Auxiliary Space: O(n), due to the stack space during recursive call. 
+*/