404. Sum of Left Leaves

Author: TBXark

404. Sum of Left Leaves.md

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+# 404. Sum of Left Leaves
+
+### 2017-03-27
+
+Find the sum of all left leaves in a given binary tree.
+
+**Example:**
+
+```
+    3
+   / \
+  9  20
+    /  \
+   15   7
+
+There are two left leaves in the binary tree, with values 9 and 15 respectively. Return 24.
+```
+
+
+
+# Solution
+
+```swift
+/**
+ * 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 Solution {
+    func sumOfLeftLeaves(_ root: TreeNode?) -> Int {
+        guard let n = root else { return 0 }
+        var layer = [n]
+        var count = 0
+        while layer.count > 0 {
+            for i in 0..<layer.count {
+                let node = layer.removeFirst()
+                if let l = node.left {
+                    if l.left == nil && l.right == nil {
+                        count += l.val
+                    }
+                    layer.append(l)
+                }
+                if let r = node.right {
+                    layer.append(r)
+                }
+            }
+        }
+        return count
+    }
+}
+```
+