[Tree] add a solution to Sum of Left Leaves

Author: Yi Gu

Tree/SumLeftLeaves.swift

@@ -0,0 +1,43 @@
+/**
+ * Question Link: https://leetcode.com/problems/sum-of-left-leaves/
+ * Primary idea: Recursion. Go to left and right and add to res if it is left leaf.
+ * 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 SumLeftLeaves {
+    func sumOfLeftLeaves(_ root: TreeNode?) -> Int {
+        guard let root = root else {
+            return 0
+        }
+        
+        var res = 0
+        helper(root.left, true, &res)
+        helper(root.right, false, &res)
+        
+        return res
+    }
+    
+    private func helper(_ node: TreeNode?, _ isLeft: Bool, _ res: inout Int) {
+        guard let node = node else {
+            return
+        }
+        if node.left == nil && node.right == nil && isLeft {
+            res += node.val
+        }
+        
+        helper(node.left, true, &res)
+        helper(node.right, false, &res)
+    }
+}