1 files changed, 52 insertions, 0 deletions

diff --git a/top-interview-questions/easy/trees/03_symmetric_tree.cc b/top-interview-questions/easy/trees/03_symmetric_tree.cc
new file mode 100644
index 0000000..bbdc52b
--- /dev/null
+++ b/top-interview-questions/easy/trees/03_symmetric_tree.cc
@@ -0,0 +1,52 @@
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    bool isSymmetricVector(const std::vector<const TreeNode*>& nodes) {
+        if (nodes.empty())     return true;
+        if (nodes.size() == 1) return true;
+        for (size_t i = 0; i < nodes.size()/2; ++i) {
+            const size_t j = nodes.size() - i - 1;
+            if ((nodes[i]  && !nodes[j]) ||
+                (!nodes[i] && nodes[j]) ||
+                (nodes[i]  && nodes[j] && (nodes[i]->val != nodes[j]->val))) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    bool isSymmetricBfs(std::vector<const TreeNode*>& current_level,
+                        std::vector<const TreeNode*>& next_level) {
+        if (current_level.empty()) {
+            return true;
+        }
+        if (!isSymmetricVector(current_level)) {
+            return false;
+        }
+        for (const TreeNode* node : current_level) {
+            if (node) {
+                next_level.push_back(node->left);
+                next_level.push_back(node->right);
+            }
+        }
+        current_level.clear(); // New next level.
+        return isSymmetricBfs(next_level, current_level);
+    }
+
+    bool isSymmetric(TreeNode* root) {
+        if (!root) return true;
+        std::vector<const TreeNode*> current_level({root});
+        std::vector<const TreeNode*> next_level;
+        return isSymmetricBfs(current_level, next_level);
+    }
+};