Merge pull request soapyigu#162 from soapyigu/Tree

[Tree] Add a solution to Path Sum III

Tree/PathSumIII.swift

@@ -0,0 +1,48 @@
+/**
+ * Question Link: https://leetcode.com/problems/path-sum-iii/
+ * Primary idea: Get path number of every node as root while iterating the tree
+ * Time Complexity: O(n^2), Space Complexity: O(1)
+ *
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     public var val: Int
+ *     public var left: TreeNode?
+ *     public var right: TreeNode?
+ *     public init(_ val: Int) {
+ *         self.val = val
+ *         self.left = nil
+ *         self.right = nil
+ *     }
+ * }
+ */
+
+class PathSumIII {
+    func pathSum(_ root: TreeNode?, _ sum: Int) -> Int {
+        guard let root = root else {
+            return 0
+        }
+
+        var res = totalPaths(root, sum)
+
+        let left = pathSum(root.left, sum)
+        let right = pathSum(root.right, sum)
+
+        return res + left + right
+    }
+
+    func totalPaths(_ root: TreeNode?, _ sum: Int) -> Int {
+        guard let root = root else {
+            return 0
+        }
+
+        var res = 0
+        if sum == root.val {
+            res += 1
+        }
+
+        res += totalPaths(root.left, sum - root.val)
+        res += totalPaths(root.right, sum - root.val)
+
+        return res
+    }
+}