107. Binary Tree Level Order Traversal II

Author: TBXark

107. Binary Tree Level Order Traversal II.md

@@ -0,0 +1,66 @@
+# 107. Binary Tree Level Order Traversal II
+
+### 2017-03-22
+
+Given a binary tree, return the *bottom-up level order* traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
+
+For example:
+Given binary tree `[3,9,20,null,null,15,7]`,
+
+```
+    3
+   / \
+  9  20
+    /  \
+   15   7
+
+```
+
+return its bottom-up level order traversal as:
+
+```
+[
+  [15,7],
+  [9,20],
+  [3]
+]
+```
+
+
+
+# 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 levelOrderBottom(_ root: TreeNode?) -> [[Int]] {
+        var res = [[Int]]()
+        guard let n = root else { return res }
+        var layer = [n]
+        while layer.count > 0 {
+            var temp = [Int]()
+            for i in 0..<layer.count {
+                let n = layer.removeFirst()
+                temp.append(n.val)
+                if let l = n.left { layer.append(l) }
+                if let r = n.right { layer.append(r) }
+            }
+            res.insert(temp, at: 0)
+        }
+        return res
+    }
+}
+```
+