101. Symmetric Tree

Author: TBXark

101. Symmetric Tree.md

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+# 101. Symmetric Tree
+
+### 2017-03-31
+
+Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
+
+For example, this binary tree `[1,2,2,3,4,4,3]` is symmetric:
+
+```
+    1
+   / \
+  2   2
+ / \ / \
+3  4 4  3
+
+```
+
+But the following `[1,2,2,null,3,null,3]` is not:
+
+```
+    1
+   / \
+  2   2
+   \   \
+   3    3
+
+```
+
+**Note:**
+Bonus points if you could solve it both recursively and iteratively.
+
+
+
+# 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 isSymmetric(_ root: TreeNode?) -> Bool {
+        func isSymmetric(left: TreeNode?, right: TreeNode?) -> Bool {
+            guard let l = left, let r = right, l.val == r.val  else {
+                return (left == nil && right == nil)
+            }
+            return isSymmetric(left: l.left, right: r.right) && isSymmetric(left: l.right, right: r.left)
+        }
+        return isSymmetric(left: root?.left, right: root?.right)
+
+    }
+
+}
+```
+