[8804who] WEEK 02 Solutions

3sum/8804who.py

@@ -0,0 +1,17 @@
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        temp = {}
+        answer = {}
+        nums.sort()
+        for i in range(len(nums)):
+            for j in range(i+1, len(nums)):
+                if -(nums[i]+nums[j]) in temp:
+                    ans = str(nums[i])+','+str(nums[j])   
+                    answer[ans] = 1
+            temp[nums[i]] = 1
+        temp_ans = []
+        for ans in answer.keys():
+            s1, s2 = ans.split(',')
+            temp_ans.append([int(s1), int(s2), -(int(s1)+int(s2))])
+        return temp_ans
+

climbing-stairs/8804who.py

@@ -0,0 +1,9 @@
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        dp = [1] * (n+1)
+
+        for i in range(2, n+1):
+            dp[i] = dp[i-1]+dp[i-2]
+
+        return dp[n]
+

product-of-array-except-self/8804who.py

@@ -0,0 +1,26 @@
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        answer = []
+
+        product1 = [0] * len(nums)
+        product2 = [0] * len(nums)
+
+        product1[0] = nums[0]
+
+        for i in range(1, len(nums)):
+            product1[i] = product1[i-1]*nums[i]
+
+        product2[-1] = nums[-1]
+
+        for i in range(len(nums)-2, -1, -1):
+            product2[i] = product2[i+1] * nums[i]
+
+        for i in range(len(nums)):
+            if i == 0:
+                answer.append(product2[1])
+            elif i == len(nums)-1:
+                answer.append(product1[-2])
+            else:
+                answer.append(product1[i-1]*product2[i+1])
+        return answer
+

valid-anagram/8804who.py

@@ -0,0 +1,5 @@
+from collections import Counter
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        return Counter(s) == Counter(t)
+

validate-binary-search-tree/8804who.py

@@ -0,0 +1,16 @@
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def isValidBST(self, root: Optional[TreeNode]) -> bool: 
+        def dfs(node, min_num, max_num):
+            if not node:
+                return True
+            if max_num <= node.val or node.val <= min_num:
+                return False       
+            return dfs(node.left, min_num, node.val) and dfs(node.right, node.val, max_num)
+        return dfs(root, float("-inf"), float("inf"))
+