[daiyongg-kim] WEEK 02 solutions

3sum/daiyongg-kim.py

@@ -0,0 +1,36 @@
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        nums.sort()
+
+        result = []
+        n = len(nums)
+
+        for i in range(n - 2):
+
+            if i > 0 and nums[i] == nums[i - 1]:
+                continue
+
+            left = i + 1  
+            right = n - 1 
+            while left < right:
+                current_sum = nums[i] + nums[left] + nums[right]
+
+                if current_sum == 0:
+                    result.append([nums[i], nums[left], nums[right]])
+
+                    while left < right and nums[left] == nums[left + 1]:
+                        left += 1
+
+                    while left < right and nums[right] == nums[right - 1]:
+                        right -= 1
+
+                    left += 1
+                    right -= 1
+
+                elif current_sum < 0:
+                    left += 1
+
+                else: 
+                    right -= 1
+
+        return result

climbing-stairs/daiyongg-kim.py

@@ -0,0 +1,13 @@
+class Solution:
+    def climbStairs(self, n: int) -> int:
+        if n == 1:
+            return 1
+
+        prev, curr = 1, 2 
+
+        for i in range(3, n+1):
+            next = prev + curr
+            prev = curr
+            curr = next
+
+        return curr

product-of-array-except-self/daiyongg-kim.py

@@ -0,0 +1,20 @@
+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+
+        size = len(nums)
+
+        #initialize array as 1 
+        result = [1] * size
+
+        left_pass = 1
+
+        for i in range(size):A
+            result[i] *= left_pass
+            left_pass *= nums[i]
+
+        right_pass = 1
+        for i in range(size-1, -1, -1):
+            result[i] *= right_pass
+            right_pass *= nums[i]
+
+        return result

valid-anagram/daiyongg-kim.py

@@ -0,0 +1,44 @@
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        first = {}
+        second = {}
+
+        for c in s:
+            if c in first:
+                first[c] += 1
+            else:
+                first[c] = 1
+
+        for c in t:
+            if c in second:
+                second[c] += 1
+            else:
+                second[c] = 1
+
+        return first == second
+
+
+# class Solution:
+#     def isAnagram(self, s: str, t: str) -> bool:
+#         first = {}
+
+#         if len(s) != len(t):
+#             return False
+
+#         for i in range(len(s)):
+#             if s[i] in first:
+#                 first[s[i]] += 1
+#             else:
+#                 first[s[i]] = 1
+
+#         for i in range(len(t)):
+#             if t[i] in first:
+#                 first[t[i]] -= 1
+#             else:
+#                 return False
+
+#         for i in first.values():
+#             if i != 0:
+#                 return False
+
+#         return True

validate-binary-search-tree/daiyongg-kim.py

@@ -0,0 +1,18 @@
+# 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 checkBST(root, min_val, max_val):
+            if not root:
+                return True
+
+            if not (min_val < root.val < max_val):
+                return False
+            return checkBST(root.left, min_val, root.val) and checkBST(root.right, root.val, max_val)
+
+        return checkBST(root, float('-inf'), float('inf'))