[doh6077] WEEK 03 solutions

combination-sum/doh6077.py

@@ -0,0 +1,20 @@
+class Solution:
+
+    def solve(self, candidates, remsum, cur, res, idx) :
+        if remsum == 0 :
+            res.append(list(cur))
+            return
+
+        if remsum < 0 or idx >= len(candidates) :
+            return
+        cur.append(candidates[idx])
+        self.solve(candidates, remsum-candidates[idx],cur,res,idx)
+        cur.pop()
+        self.solve(candidates,remsum,cur,res,idx+1)
+
+
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        cur = []
+        res = []
+        self.solve(candidates,target,cur,res,0)
+        return res

decode-ways/doh6077.py

@@ -0,0 +1,20 @@
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        if s[0] == '0':
+            return 0
+        n = len(s)
+        cur = 1
+        pre1 = pre2 = 1
+        for i in range(2, n+1):
+            one = int(s[i-1])
+            two = int(s[i-2:i])
+            cur = 0
+            if 1 <= one <= 9:
+                cur += pre1
+            if 10 <= two <= 26:
+                cur += pre2
+
+            pre2 = pre1
+            pre1 = cur
+
+        return cur

maximum-subarray/doh6077.py

@@ -0,0 +1,13 @@
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:            
+        res = nums[0]
+        total = 0
+
+        for n in nums:
+            if total < 0:
+                total = 0
+
+            total += n
+            res = max(res, total)
+
+        return res

number-of-1-bits/doh6077.py

@@ -0,0 +1,7 @@
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        ans = 0 
+        while n != 0:
+            ans += 1
+            n = n & ( n-1)
+        return ans 

valid-palindrome/doh6077.py

@@ -0,0 +1,16 @@
+import re
+# two pointers 
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        # remove all alphanumeric characters and convert all uppercase letters into lowercase letters
+        cleaned_s = re.sub(r'[^a-zA-Z0-9]', '', s).lower()
+        n = len(cleaned_s)
+        left, right = 0, n -1
+        while left < right: 
+            if cleaned_s[left] == cleaned_s[right]:
+                left += 1 
+                right -= 1
+            else:
+                return False 
+
+        return True