Create 617_merge_two_binary_trees.md

Author: Joseph Luce

leetcode/easy/617_merge_two_binary_trees.md

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+# 617. Merge Two Binary Trees
+
+## Recursion Solution
+
+- Runtime: O(N)
+- Space: O(1) (Assuming creating the result is not extra space)
+- N = Number of elements in both trees
+
+By traversing both trees together, the solution is much simpler. 
+Use one of the trees as your primary merge tree during the recursion and create new nodes when the primary doesn't have one and the secondary tree does.
+With recursion, you need to create the new nodes when you are the parent before you perform a function call into that node.
+You cannot create the new node if the node doesn't exist after the call.
+
+```
+class Solution:
+    def mergeTrees(self, t1: TreeNode, t2: TreeNode) -> TreeNode:
+        def merge_tree_helper(T1, T2):
+            if T1 is None or T2 is None:
+                return
+            T1.val += T2.val #T1 is the primary tree
+            if T2.left is not None and T1.left is None:
+                T1.left = TreeNode(0)
+            if T2.right is not None and T1.right is None:
+                T1.right = TreeNode(0)
+            merge_tree_helper(T1.left, T2.left)
+            merge_tree_helper(T1.right, T2.right)
+        
+        if t1 is None:
+            return t2
+        merge_tree_helper(t1, t2)
+        return t1
+```