diff --git a/11_Binary-Search/06. Valid Triangle Number.py b/11_Binary-Search/06. Valid Triangle Number.py
new file mode 100644
index 00000000..0d809540
--- /dev/null
+++ b/11_Binary-Search/06. Valid Triangle Number.py	
@@ -0,0 +1,22 @@
+# https://leetcode.com/problems/valid-triangle-number/
+# https://youtu.be/PqEiJDdt3S4
+
+class Solution:
+    def triangleNumber(self, nums: List[int]) -> int:
+        nums.sort()
+        res = 0
+        
+        for i in range(len(nums)-1, 1, -1):
+            r = i - 1
+            l = 0
+            while l < r:
+                if nums[l] + nums[r] > nums[i]:
+                    res += (r - l)
+                    r -= 1
+                else:
+                    l += 1
+        
+        return res
+
+# Time: O(N^2)
+# Space: O(1)
diff --git a/16_String/03. Longest Common Prefix.py b/16_String/03. Longest Common Prefix.py
new file mode 100644
index 00000000..554ddb26
--- /dev/null
+++ b/16_String/03. Longest Common Prefix.py	
@@ -0,0 +1,13 @@
+# https://leetcode.com/problems/longest-common-prefix/
+# https://youtu.be/0sWShKIJoo4
+
+class Solution:
+    def longestCommonPrefix(self, strs: List[str]) -> str:
+        res = ""
+        for i in range(len(strs[0])):
+            for st in strs:
+                if i == len(st) or strs[0][i] != st[i]:
+                    return res
+            res += strs[0][i]
+        
+        return res
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