5 files changed, 197 insertions, 0 deletions

diff --git a/top-interview-questions/easy/trees/01_maximum_depth_of_binary_tree.cc b/top-interview-questions/easy/trees/01_maximum_depth_of_binary_tree.cc
new file mode 100644
index 0000000..4d13af7
--- /dev/null
+++ b/top-interview-questions/easy/trees/01_maximum_depth_of_binary_tree.cc
@@ -0,0 +1,20 @@
+/**
+ * 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:
+    int max(int a, int b) { return a > b ? a : b; }
+
+    int maxDepth(TreeNode* root) {
+        if (!root) return 0;
+        return 1 + max(maxDepth(root->left), maxDepth(root->right));
+    }
+};
diff --git a/top-interview-questions/easy/trees/02_valid_binary_search_tree.cc b/top-interview-questions/easy/trees/02_valid_binary_search_tree.cc
new file mode 100644
index 0000000..a071daf
--- /dev/null
+++ b/top-interview-questions/easy/trees/02_valid_binary_search_tree.cc
@@ -0,0 +1,54 @@
+/**
+ * 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 isValidBSTRec(TreeNode* root, int* min, int* max) {
+        if (root->left && root->right) {
+            int leftMin, leftMax, rightMin, rightMax;
+            if (!isValidBSTRec(root->left,  &leftMin,  &leftMax) ||
+                !isValidBSTRec(root->right, &rightMin, &rightMax)) {
+                return false;
+            }
+            *min = std::min(std::min(leftMin, rightMin), root->val);
+            *max = std::max(std::max(leftMax, rightMax), root->val);
+            return (leftMax < root->val) && (root->val < rightMin);
+        }
+        else if (root->left) {
+            int leftMin, leftMax;
+            if (!isValidBSTRec(root->left, &leftMin, &leftMax)) {
+                return false;
+            }
+            *min = std::min(leftMin, root->val);
+            *max = std::max(leftMax, root->val);
+            return (leftMax < root->val);
+        }
+        else if (root->right) {
+            int rightMin, rightMax;
+            if (!isValidBSTRec(root->right, &rightMin, &rightMax)) {
+                return false;
+            }
+            *min = std::min(rightMin, root->val);
+            *max = std::max(rightMax, root->val);
+            return (root->val < rightMin);
+        }
+        else {
+            *min = *max = root->val;
+            return true;
+        }
+    }
+
+    bool isValidBST(TreeNode* root) {
+        if (!root) return true;
+        int minVal, maxVal;
+        return isValidBSTRec(root, &minVal, &maxVal);
+    }
+};
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);
+    }
+};
diff --git a/top-interview-questions/easy/trees/04_binary_tree_level_order_traversal.cc b/top-interview-questions/easy/trees/04_binary_tree_level_order_traversal.cc
new file mode 100644
index 0000000..9c506ab
--- /dev/null
+++ b/top-interview-questions/easy/trees/04_binary_tree_level_order_traversal.cc
@@ -0,0 +1,44 @@
+/**
+ * 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:
+    void bfs(vector<vector<int>>& levels,
+             vector<const TreeNode*>& current_level,
+             vector<const TreeNode*>& next_level) {
+        if (current_level.empty()) {
+            return;
+        }
+        levels.push_back({});
+        vector<int>& level = levels.back();
+        level.reserve(current_level.size());
+        for (const TreeNode* node : current_level) {
+            level.push_back(node->val);
+            if (node->left) {
+                next_level.push_back(node->left);
+            }
+            if (node->right) {
+                next_level.push_back(node->right);
+            }
+        }
+        current_level.clear(); // New next level.
+        bfs(levels, next_level, current_level);
+    }
+
+    vector<vector<int>> levelOrder(TreeNode* root) {
+        if (!root) return {};
+        vector<vector<int>> levels;
+        vector<const TreeNode*> current_level({root});
+        vector<const TreeNode*> next_level;
+        bfs(levels, current_level, next_level);
+        return levels;
+    }
+};
diff --git a/top-interview-questions/easy/trees/05_convert_sorted_array_to_binary_search_tree.cc b/top-interview-questions/easy/trees/05_convert_sorted_array_to_binary_search_tree.cc
new file mode 100644
index 0000000..4882a2a
--- /dev/null
+++ b/top-interview-questions/easy/trees/05_convert_sorted_array_to_binary_search_tree.cc
@@ -0,0 +1,27 @@
+/**
+ * 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:
+    // 'end' is one past the end.
+    TreeNode* makeTree(const vector<int>& nums, size_t start, size_t end) {
+        if (start >= end) return nullptr;
+        const size_t middle = start + (end - start) / 2;
+        TreeNode* root = new TreeNode(nums[middle]);
+        root->left  = makeTree(nums, start, middle);
+        root->right = makeTree(nums, middle+1, end);
+        return root;
+    }
+
+    TreeNode* sortedArrayToBST(vector<int>& nums) {
+        return makeTree(nums, 0, nums.size());
+    }
+};