Create 112_path_sum.md

Author: Joseph Luce

leetcode/easy/112_path_sum.md

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+# 112. Path Sum
+
+## Recursive Solution
+- Runtime: O(N)
+- Space: O(N)
+- N = Number of nodes in tree
+
+Keeping track of the sum as we go down the tree is one way. 
+So that when we reach a leaf node (no children), we can check the current sum against the target sum.
+Since its recursion, the current sum before the recursion call would be used so we don't need to subtract from the current sum.
+We let recursion do that for us.
+
+Since we will be visiting all the nodes, the runtime is O(N). 
+In the worst case, we would have to have every node on the stack if the whole tree was just a linked list, space would be O(N).
+
+```
+class Solution:
+    def hasPathSum(self, root: TreeNode, sum: int) -> bool:
+        def has_path_sum_helper(root, curr_sum, target):
+            if root is None:
+                return False
+            curr_sum += root.val
+            if root.left is None and root.right is None:
+                return curr_sum == target
+            return has_path_sum_helper(root.left, curr_sum, target) \
+                or has_path_sum_helper(root.right, curr_sum, target)
+        
+        return has_path_sum_helper(root, 0, sum)
+```