[Tree] add Solution to PathSum, refactor code style

Tree/PathSum.swift

@@ -0,0 +1,30 @@
+/**
+ * Question Link: https://leetcode.com/problems/path-sum/
+ * Primary idea: recursion
+ * Time Complexity: O(n), 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 PathSum {
+    func hasPathSum(root: TreeNode?, _ sum: Int) -> Bool {
+        if root == nil {
+            return false
+        }
+        if sum == root!.val && root!.left == nil && root!.right == nil {
+            return true
+        }
+
+        return hasPathSum(root!.left, sum - root!.val) || hasPathSum(root!.right, sum - root!.val)
+    }
+}

Tree/SameTree.swift

@@ -1,6 +1,6 @@
 /**
- * Question Link: https://leetcode.com/problems/same-tree
- * Primary solution: recursion
+ * Question Link: https://leetcode.com/problems/same-tree/
+ * Primary idea: recursion
  * Time Complexity: O(n), Space Complexity: O(1)
  *
  * Copyright © 2016 YiGu. All rights reserved.
@@ -20,13 +20,13 @@
 
 class SameTree {
     func isSameTree(p: TreeNode?, _ q: TreeNode?) -> Bool {
-        if (p == nil && q == nil) {
+        if p == nil && q == nil {
             return true
         }
-        if (p == nil || q == nil || p?.val != q?.val) {
+        if p == nil || q == nil || p!.val != q!.val {
             return false
         }
 
-        return isSameTree(p?.left, q?.left) && isSameTree(p?.right, q?.right)
+        return isSameTree(p!.left, q!.left) && isSameTree(p!.right, q!.right)
     }
 }

Tree/SymmetricTree.swift

@@ -1,6 +1,6 @@
 /**
  * Question Link: https://leetcode.com/problems/symmetric-tree/
- * Primary solution: recursion
+ * Primary idea: recursion
  * Time Complexity: O(n), Space Complexity: O(1)
  * 
  * Definition for a binary tree node.
@@ -17,21 +17,21 @@
  *
  */
 
-class Solution {
+class SymmetricTree {
     func isSymmetric(root: TreeNode?) -> Bool {
-        if (root == nil) {
+        if root == nil {
             return true
         }
         return helper(root!.left, root!.right)
     }
 
     func helper(p: TreeNode?, _ q:TreeNode?) -> Bool {
-        if (p == nil && q == nil) {
+        if p == nil && q == nil {
             return true
         }
-        if (p == nil || q == nil || p?.val != q?.val) {
+        if p == nil || q == nil || p!.val != q!.val {
             return false
         }
-        return helper(p?.left, q?.right) && helper(p?.right, q?.left)
+        return helper(p!.left, q!.right) && helper(p!.right, q!.left)
     }
 }