diff --git a/best-time-to-buy-and-sell-stock/wclee2265.cpp b/best-time-to-buy-and-sell-stock/wclee2265.cpp
new file mode 100644
index 000000000..7a0b65755
--- /dev/null
+++ b/best-time-to-buy-and-sell-stock/wclee2265.cpp
@@ -0,0 +1,17 @@
+// https://leetcode.com/problems/best-time-to-buy-and-sell-stock/
+// Time Complexity : O(n)
+// Space Complexity : O(1)
+
+class Solution {
+public:
+    int maxProfit(vector<int>& prices) {
+        int n = prices.size();
+        int minimum = prices[0];
+        int ans = 0;
+        for(int i=1; i<n; i++) {
+            ans = max(ans, prices[i] - minimum);
+            minimum = min(minimum, prices[i]);
+        }
+        return ans;
+    }
+};
diff --git a/contains-duplicate/wclee2265.cpp b/contains-duplicate/wclee2265.cpp
new file mode 100644
index 000000000..a16c744a2
--- /dev/null
+++ b/contains-duplicate/wclee2265.cpp
@@ -0,0 +1,15 @@
+// https://leetcode.com/problems/contains-duplicate/
+// Time Complexity : O(n)
+// Space Complexity : O(n)
+
+class Solution {
+public:
+    bool containsDuplicate(vector<int>& nums) {
+        unordered_map<int, bool> hm;
+        for(int x : nums) {
+            if(hm[x]) return true;
+            hm[x] = true;
+        }
+        return false;        
+    }
+};
diff --git a/two-sum/wclee2265.cpp b/two-sum/wclee2265.cpp
new file mode 100644
index 000000000..b23c30432
--- /dev/null
+++ b/two-sum/wclee2265.cpp
@@ -0,0 +1,27 @@
+// https://leetcode.com/problems/two-sum/
+// Time Complexity : O(nlogn)
+// Space Complexity : O(n)
+
+class Solution {
+public:
+    vector<int> twoSum(vector<int>& nums, int target) {
+        int n = nums.size();
+        vector<pair<int, int> > numsIndices;
+
+        for(int i=0; i<n; i++){
+            numsIndices.push_back({nums[i], i});
+        }
+
+        sort(numsIndices.begin(), numsIndices.end());
+        int i, j = n-1;
+        for(i=0; i<n; i++) {
+            while(numsIndices[i].first + numsIndices[j].first > target) j--;
+            if(numsIndices[i].first + numsIndices[j].first == target) break;
+        }
+
+        vector<int> ans;
+        ans.push_back(numsIndices[i].second);
+        ans.push_back(numsIndices[j].second);
+        return ans;
+    }
+};
diff --git a/valid-anagram/wclee2265.cpp b/valid-anagram/wclee2265.cpp
new file mode 100644
index 000000000..2f2d65e76
--- /dev/null
+++ b/valid-anagram/wclee2265.cpp
@@ -0,0 +1,18 @@
+// https://leetcode.com/problems/valid-anagram/
+// Time Complexity : O(n)
+// Space Complexity : O(1)
+
+class Solution {
+public:
+    bool isAnagram(string s, string t) {
+        int cnt1[26]={0,}, cnt2[26] = {0,};
+        for(char c : s) cnt1[c-'a']++;
+        for(char c : t) cnt2[c-'a']++;
+
+        for(int i=0; i<26; i++) {
+            if(cnt1[i] != cnt2[i]) return false;
+        }
+
+        return true;
+    }
+};
diff --git a/valid-palindrome/wclee2265.cpp b/valid-palindrome/wclee2265.cpp
new file mode 100644
index 000000000..a5c98bd63
--- /dev/null
+++ b/valid-palindrome/wclee2265.cpp
@@ -0,0 +1,21 @@
+// https://leetcode.com/problems/valid-palindrome/
+// Time Complexity : O(n)
+// Space Complexity : O(n)
+
+class Solution {
+public:
+    bool isPalindrome(string s) {
+        string ns;
+        for(char c : s){
+            if(c >= '0' && c <= '9') ns += c;
+            else if(c >= 'A' && c <= 'Z') ns += (c + 32);
+            else if(c >= 'a' && c <= 'z') ns += c;
+        }
+
+        int n = ns.size();
+        for(int i=0; i<n/2; i++) {
+            if(ns[i] != ns[n-i-1]) return false;
+        }
+        return true;
+    }
+};