[jinah92] Week 3

combination-sum/jinah92.py

@@ -0,0 +1,19 @@
+# O(T) time, O(C^T) space
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        results, nums = [], []
+
+        def dfs(start, total):
+            if total > target:
+                return
+            if total == target:
+                results.append(nums[:])
+            for i in range(start, len(candidates)):
+                num = candidates[i]
+                nums.append(num)
+                dfs(i, total + num)
+                nums.pop()
+
+        dfs(0, 0)
+
+        return results

maximum-subarray/jinah92.py

@@ -0,0 +1,14 @@
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        num_set = {}
+        result = -math.inf
+
+        for idx, num in enumerate(nums):
+            if idx == 0:
+                num_set[idx] = max(nums[0], result)
+            else:
+                num_set[idx] = max(num, num_set[idx-1] + num)
+            tmp_sum = num_set[idx]
+            result = max(result, tmp_sum)
+
+        return result

product-of-array-except-self/jinah92.py

@@ -0,0 +1,21 @@
+# O(n) time, O(n) space
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        non_zero_product = math.prod(filter(lambda x: x != 0, nums))
+        raw_product = math.prod(nums)
+        zero_total = nums.count(0)
+
+        result = []
+
+        for num in nums:
+            if zero_total > 1:
+                result.append(0)
+            elif zero_total == 1:
+                if num == 0:
+                    result.append(non_zero_product)
+                else:
+                    result.append(raw_product)
+            else:
+                result.append(raw_product // num)
+
+        return result

reverse-bits/jinah92.py

@@ -0,0 +1,14 @@
+# O(1) time, O(1) space
+class Solution:
+    def reverseBits(self, n: int) -> int:
+        stack = []
+        while len(stack) < 32:
+            stack.append(n % 2)
+            n //=2
+
+        result, scale = 0, 1
+        while stack:
+            result += stack.pop() * scale
+            scale *= 2
+
+        return result

two-sum/jinah92.py

@@ -0,0 +1,12 @@
+# O(n) time, O(n) space
+
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        num_set = {}
+
+        for idx, num in enumerate(nums):
+            other_num = target - num
+            if other_num in num_set:
+                return [idx, num_set[other_num]]
+            else:
+                num_set[num] = idx