diff --git a/3sum/hwi-middle.cpp b/3sum/hwi-middle.cpp
new file mode 100644
index 0000000000..c77a7db04a
--- /dev/null
+++ b/3sum/hwi-middle.cpp
@@ -0,0 +1,41 @@
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+        vector<vector<int>> res;
+
+        sort(nums.begin(), nums.end());
+
+        for (int i = 0; i < nums.size() && nums[i] <= 0; ++i)
+        {
+            if (i != 0 && nums[i - 1] == nums[i]) 
+            {
+                continue;
+            }
+
+            int l = i + 1;
+            int r = nums.size() - 1;
+            while (l < r) 
+            {
+                int sum = nums[i] + nums[l] + nums[r];
+                if (sum < 0) 
+                {
+                    ++l;
+                }
+                else if (sum > 0) 
+                {
+                    --r;
+                }
+                else 
+                {
+                    res.push_back({nums[i], nums[l++], nums[r--]});
+                    while (l < r && nums[l] == nums[l - 1])
+                    {
+                        ++l;
+                    }
+                }
+            }
+        }
+
+        return res;
+    }
+};
diff --git a/climbing-stairs/hwi-middle.cpp b/climbing-stairs/hwi-middle.cpp
new file mode 100644
index 0000000000..e33c45456c
--- /dev/null
+++ b/climbing-stairs/hwi-middle.cpp
@@ -0,0 +1,16 @@
+class Solution {
+public:
+    int climbStairs(int n) {
+        int p = 1;
+        int pp = 0;
+        int cur = 0;
+        for (int i = 0; i < n; ++i)
+        {
+            cur = p + pp;
+            pp = p;
+            p = cur;
+        }
+
+        return cur;
+    }
+};
diff --git a/product-of-array-except-self/hwi-middle.cpp b/product-of-array-except-self/hwi-middle.cpp
new file mode 100644
index 0000000000..0b2fb4a470
--- /dev/null
+++ b/product-of-array-except-self/hwi-middle.cpp
@@ -0,0 +1,22 @@
+class Solution {
+public:
+    vector<int> productExceptSelf(vector<int>& nums) {
+        int n = nums.size();
+        vector<int> ans(n);
+
+        ans[0] = 1;
+        for (int i = 1; i < n; i++)
+        {
+            ans[i] = ans[i-1] * nums[i-1];
+        }
+
+        int suffix = 1;
+        for (int i = n-1; i >= 0; i--) 
+        {
+            ans[i] *= suffix;
+            suffix *= nums[i];
+        }
+
+        return ans;
+    }
+};
diff --git a/valid-anagram/hwi-middle.cpp b/valid-anagram/hwi-middle.cpp
new file mode 100644
index 0000000000..9c4f9be518
--- /dev/null
+++ b/valid-anagram/hwi-middle.cpp
@@ -0,0 +1,29 @@
+class Solution {
+public:
+    bool isAnagram(string s, string t) {
+        if (s.size() != t.size())
+        {
+            return false;
+        }
+
+        int cnt_s[26];
+        int cnt_t[26];
+        
+        int len = s.size();
+        for (int i = 0; i < len; ++i)
+        {
+            cnt_s[s[i] - 'a']++;
+            cnt_t[t[i] - 'a']++;
+        }
+
+        for (int i = 0; i < 26; ++i)
+        {
+            if(cnt_s[i] != cnt_t[i])
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+};
diff --git a/validate-binary-search-tree/hwi-middle.cpp b/validate-binary-search-tree/hwi-middle.cpp
new file mode 100644
index 0000000000..eaaeda8f36
--- /dev/null
+++ b/validate-binary-search-tree/hwi-middle.cpp
@@ -0,0 +1,46 @@
+/**
+ * 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 isValidBST(TreeNode* root) {
+        vector<int> v;
+        v.reserve(1e4);
+        
+        solve(root, v);
+
+        int len = v.size();
+        for (int i = 0; i < len - 1; ++i)
+        {
+            if (v[i] >= v[i + 1])
+            {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    void solve(TreeNode* root, vector<int>& v)
+    {
+        if (root->left != nullptr)
+        {
+            solve(root->left, v);
+        }
+
+        v.push_back(root->val);
+
+        if (root->right != nullptr)
+        {
+            solve(root->right, v);
+        }
+    }
+};