Merge pull request #1810 from prograsshopper/main

[prograsshopper] Week 3 solution

Author: prograsshopper

combination-sum/prograsshopper.py

@@ -0,0 +1,17 @@
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        # Time complexity: O(n ^ (T / m))
+        result = []
+
+        def dfs(remain_sum, index, path):
+            if remain_sum < 0:
+                return
+            if remain_sum == 0:
+                result.append(path)
+                return
+
+            for i in range(index, len(candidates)):
+                dfs(remain_sum - candidates[i], i, path + [candidates[i]])
+
+        dfs(target, 0, [])
+        return result

decode-ways/prograsshopper.py

@@ -0,0 +1,19 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        # sol 1: Time complexity O(n)
+        dp = {len(s): 1}
+
+        def dfs(start):
+            if start in dp:
+                return dp[start]
+            if s[start] == "0":
+                return 0
+            result = None
+            if start + 1 < len(s) and int(s[start:start+2]) < 27:
+                result = dfs(start+1) + dfs(start+2)
+            else:
+                result = dfs(start+1)
+            dp[start] = result
+            return result
+
+        return dfs(0)

number-of-1-bits/prograsshopper.py

@@ -0,0 +1,10 @@
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        # Time complexity: O(log n)
+        output = 1
+        while n > 1:
+            remain = n % 2
+            n = n // 2
+            if remain == 1:
+                output += 1
+        return output

valid-palindrome/prograsshopper.py

@@ -0,0 +1,18 @@
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        formatted_string = "".join(elem.lower() for elem in s if elem.isalnum())
+
+        # sol 1
+        # Time complexity: O(n)
+        return formatted_string == formatted_string[::-1]
+
+        # sol 2
+        # Time complexity: O(n)
+        left = 0
+        right = len(formatted_string) - 1
+        while left < right:
+            if formatted_string[left] != formatted_string[right]:
+                return False
+        left += 1
+        right -= 1
+        return True