Week3

climbing-stairs/joon.py

@@ -0,0 +1,24 @@
+from typing import Dict
+
+
+class Solution:
+    # Time: O(n)
+    # Space: O(n)
+    # ---
+    # 1. Prepare a table for caching. Initially put trivial answers for n = 1, 2.
+    # Space: O(n)
+    cacheTableOfAnswerByN: Dict[int, int] = {1: 1, 2: 2}
+
+    def climbStairs(self, n: int) -> int:
+        # 2. Use caching.
+        # Time: O(1)
+        try: return self.cacheTableOfAnswerByN[n]
+        except KeyError:
+            # 3. Simply, the answers follow Fibonacci sequence. Use recursion.
+            # O(n)
+            answerBeforeTwoSteps = self.climbStairs(n - 2)
+            # 4. Cache the answer during recursion.
+            self.cacheTableOfAnswerByN[n - 2] = answerBeforeTwoSteps
+            answerBeforeOneStep = self.climbStairs(n - 1)
+            self.cacheTableOfAnswerByN[n - 1] = answerBeforeOneStep
+            return answerBeforeTwoSteps + answerBeforeOneStep

combination-sum/joon.py

@@ -0,0 +1,34 @@
+from typing import List
+from typing import Set
+from typing import Tuple
+
+
+class Solution:
+    target: int
+    candidates: List[int]
+    answers: Set[Tuple[int]]
+
+    # Time: O(k^N)
+    # Space: O(N^2)
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        self.target = target
+        self.candidates = candidates
+        self.answers = set()
+        self.findAnswers(0, [])
+        return list(self.answers)
+
+    def findAnswers(self, currentSum: int, nums: List[int]):
+        assert currentSum < self.target, "Given sum should be smaller than the target."
+        for num in self.candidates:
+            if currentSum + num < self.target:
+                # 1. Continue finding.
+                # Time: O(k^N)
+                # Space: O(N^2). Max total size of all "nums" = 1 + 2 + ... + N.
+                self.findAnswers(currentSum + num, nums + [num])
+            if currentSum + num > self.target:
+                # 2. Stop finding.
+                continue
+            if currentSum + num == self.target:
+                # 3. Add the answer.
+                self.answers.add(tuple(sorted(list(nums + [num]))))
+        return

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

@@ -0,0 +1,28 @@
+from typing import List
+from functools import reduce
+
+
+class Solution:
+    # Time: O(n)
+    # Space: O(n)
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        size: int = len(nums)
+        if size == 1:
+            return [1]
+        # 1. Cut the array at the middle.
+        # Time: O(1)
+        # Space: O(n)
+        indexToCut: int = len(nums) // 2
+        left: List[int] = nums[0:indexToCut]
+        right: List[int] = nums[indexToCut:size]
+
+        # 2. Divide, conquer, and merge. But no benefit in complexity.
+        # Time: O(n)
+        # Space: O(n)
+        return self.multiplyToAll(self.productExceptSelf(left), self.calculateProductAll(right)) + self.multiplyToAll(self.productExceptSelf(right), self.calculateProductAll(left))
+
+    def calculateProductAll(self, nums: List[int]) -> int:
+        return reduce(lambda x, y: x * y, nums)
+
+    def multiplyToAll(self, nums: List[int], multiplier: int) -> List[int]:
+        return list(map(lambda num: multiplier * num, nums))

two-sum/joon.py

@@ -0,0 +1,28 @@
+from typing import List
+from typing import Dict
+
+
+class Solution:
+    # Time: O(n)
+    # Space: O(n)
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        # 1. Make a map {target - num : index of num} with the list nums.
+        # Time: O(n)
+        # Space: O(n)
+        mapOfSubtractedValueToIndex: Dict[int, int] = ({
+            target - num: index for index, num in enumerate(nums)
+        })
+
+        # 2. ForEach num in nums, get value from the map.
+        # Time: O(n)
+        # Space: O(n)
+        result: List[int] = None
+        for indexOfNum, num in enumerate(nums):
+            try:
+                indexOfSubtractedValue = mapOfSubtractedValueToIndex[num]
+                if indexOfSubtractedValue == indexOfNum:
+                    continue
+                # 3. Return the index of num in nums and the value from the map.
+                return [indexOfNum, indexOfSubtractedValue]
+            except KeyError: 
+                continue