chore(Java): add left leaf sum binary tree (#839)

algorithms/Java/README.md

@@ -101,6 +101,7 @@
 - [Min Heap](trees/MinHeap.java)
 - [Check Tree Traversal](trees/check-tree-traversal.java)
 - [Random Node of a binary tree](tree-random-node.java)
+- [Left Leaf Sum of a Binary tree](trees/Left-Leaf-Sum.java)
 
 ## Backtracking
 

algorithms/Java/trees/Left-Leaf-Sum.java

@@ -0,0 +1,95 @@
+package Trees.Left_Leaf_Sum;
+/* Problem Statement: Given the root of a binary tree, return the sum of all left leaves. */
+
+//class to create Tree
+class TreeNode{
+	  int data;
+	  TreeNode left,right;
+
+	  TreeNode(int data){
+	    this.data = data;
+	    this.left = this.right = null;
+	  }
+	}
+class Tree{
+	TreeNode root;
+	
+	public Tree() {
+		root=null;
+	}
+}
+public class LeftLeafSum {
+	
+	 //method to check if a node is leaf or not
+	static boolean isLeaf(TreeNode root)
+    {
+        if (root == null)   return false;
+        if (root.left == null && root.right == null)    
+        return true;
+        
+        return false;
+    }
+	
+	//returns the sum of left leaves
+        public static int leftsum(TreeNode root){
+        
+        int leftLeavesSum=0;
+        
+        //to avoid null pointer exception
+        if(root!=null)
+        {
+        
+        //if a node's left is a leaf then add its value	
+        if( isLeaf(root.left) )
+        leftLeavesSum+=root.left.data;
+        
+        //else go to left
+        else
+        leftLeavesSum+=leftsum(root.left);
+        
+        //and finally go to right
+        leftLeavesSum+=leftsum(root.right);
+        }
+        
+        return leftLeavesSum;
+        
+        
+        
+        
+  }
+	
+public static void main(String[] args) {
+	Tree tree = new Tree();
+    tree.root = new TreeNode(1);
+    tree.root.left = new TreeNode(2);
+    tree.root.right = new TreeNode(3);
+    tree.root.left.left = new TreeNode(7);
+    tree.root.left.right = new TreeNode(8);
+    tree.root.right.left = new TreeNode(81);
+    tree.root.right.right= new TreeNode(75);
+    
+    
+    int sumOfLeftLeaves = leftsum(tree.root);
+    System.out.println("sum of left leaves is "+sumOfLeftLeaves);   
+}
+}
+
+/*
+
+Eg. Input:-
+                                1
+		              /   \
+		            2      3
+		           / \    / \		
+                          7   8  81  75  
+Output:- 88
+
+Explanation:-there are two left leaves(nodes which have no child) 7 and 81 respectively in the tree and their sum is 88
+
+Time Complexity:- O(N), where n is number of nodes in Binary Tree.
+
+
+Code Description:- A method isLeaf() which returns a boolean, is created to check if a node is leaf or not, is yes then add its
+left.data, if not then recursively go to its left and then its right.
+
+*/