Merge pull request #844 from HodaeSsi/main

[HodaeSsi] Week4

Author: SamTheKorean

coin-change/HodaeSsi.py

@@ -0,0 +1,17 @@
+# space complexity: O(n) (여기서 n은 amount)
+# time complexity: O(n * m) (여기서 n은 amount, m은 coins의 길이)
+from typing import List
+
+
+class Solution:
+    def coinChange(self, coins: List[int], amount: int) -> int:
+        dp = [float('inf')] * (amount + 1)
+        dp[0] = 0
+        
+        for i in range(1, amount + 1):
+            for coin in coins:
+                if coin <= i:
+                    dp[i] = min(dp[i], dp[i - coin] + 1)
+        
+        return dp[amount] if dp[amount] != float('inf') else -1
+    

merge-two-sorted-lists/HodaeSsi.py

@@ -0,0 +1,22 @@
+from typing import Optional
+
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+        
+class Solution:
+	def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+		if not list1:
+			return list2
+		if not list2:
+			return list1
+
+		if list1.val < list2.val:
+			list1.next = self.mergeTwoLists(list1.next, list2)
+			return list1
+		else:
+			list2.next = self.mergeTwoLists(list1, list2.next)
+			return list2
+

missing-number/HodaeSsi.py

@@ -0,0 +1,9 @@
+class Solution:
+	def missingNumber(self, nums: List[int]) -> int:
+		n = len(nums)
+		expected_sum = (n ** 2 + n) // 2
+		
+		actual_sum = sum(nums)
+		
+		return expected_sum - actual_sum
+

palindromic-substrings/HodaeSsi.py

@@ -0,0 +1,19 @@
+# space complexity: O(n^2)
+# time complexity: O(n^2) (*exactly, n^2 / 2)
+class Solution:
+	def countSubstrings(self, s: str) -> int:
+		dp = [[False] * len(s) for _ in range(len(s))] # dp[i][j] = True if s[i:j+1] is a palindrome
+		for i in range(len(s)):
+			for j in range(i, -1, -1):
+				if i == j:
+					dp[j][i] = True
+					continue
+				if i - j == 1:
+					dp[j][i] = s[i] == s[j]
+					continue
+				if s[i] == s[j]:
+						if dp[j+1][i-1]:
+							dp[j][i] = True
+		
+		return sum([sum(row) for row in dp])
+

word-search/HodaeSsi.py

@@ -0,0 +1,29 @@
+class Solution:
+	def dfs (self, board, word, visited, y, x, word_idx):
+		if word_idx == len(word):
+			return True
+		
+		if y < 0 or y >= len(board) or x < 0 or x >= len(board[0]) or visited[y][x] or board[y][x] != word[word_idx]:
+			return False
+		
+		visited[y][x] = True
+		for dy, dx in [(1, 0), (-1, 0), (0, -1), (0, 1)]:
+			if self.dfs(board, word, visited, y + dy, x + dx, word_idx + 1):
+				return True
+		visited[y][x] = False
+		return False
+
+	def exist(self, board: List[List[str]], word: str) -> bool:
+		visited = [[False for _ in range(len(board[0]))] for _ in range(len(board))]
+
+		# find fisrt letter in board
+		for y in range(len(board)):
+			for x in range(len(board[0])):
+				if board[y][x] == word[0]:
+					visited[y][x] = True
+					for dy, dx in [(1, 0), (-1, 0), (0, -1), (0, 1)]:
+						if self.dfs(board, word, visited, y + dy, x + dx, 1):
+							return True
+					visited[y][x] = False
+		return False
+