Merge pull request #810 from jinah92/main

[jinah92] Week 4

Author: jinah92

coin-change/jinah92.py

@@ -0,0 +1,10 @@
+# O(C*A) times, O(A) spaces
+class Solution:
+    def coinChange(self, coins: List[int], amount: int) -> int:
+       dp = [0] + [amount + 1] * amount
+
+       for coin in coins:
+            for i in range(coin, amount + 1):
+                dp[i] = min(dp[i], dp[i-coin]+1)
+
+       return dp[amount] if dp[amount] < amount + 1 else -1

merge-two-sorted-lists/jinah92.py

@@ -0,0 +1,18 @@
+# O(m+n) times, O(1) spaces
+class Solution:
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        dummy = ListNode(None)
+        node = dummy
+
+        while list1 and list2:
+            if list1.val < list2.val:
+                node.next = list1
+                list1 = list1.next
+            else:
+                node.next = list2
+                list2 = list2.next
+
+            node = node.next
+        
+        node.next = list1 or list2
+        return dummy.next

missing-number/jinah92.py

@@ -0,0 +1,10 @@
+# O(n) times, O(n) spaces
+class Solution:
+    def missingNumber(self, nums: List[int]) -> int:
+        nums_keys = set(nums)
+        
+        for i in range(len(nums)):
+            if not i in nums_keys:
+                return i
+        
+        return len(nums)

palindromic-substrings/jinah92.py

@@ -0,0 +1,39 @@
+# O(N^3) times, O(1) spaces
+# 내부 while문의 관계가 외부 while문의 sub_str_len에 따라 반복횟수가 줄어드므로, 1+2+...N = N(N-1)/2 = O(N2) 시간 소요
+# 추라고 내부 while에서 sub_str_len에 따라 s가 인덱싱되므로 최대 O(N) 시간이 소요
+# 최종적으로 O(N^2 * N) = O(N^3)이 소요됨
+class Solution:
+    def countSubstrings(self, s: str) -> int:
+        sub_str_len = 1
+        result = 0
+
+        while sub_str_len <= len(s):
+            start_idx = 0
+            while start_idx + sub_str_len <= len(s):
+                sub_str = s[start_idx:start_idx+sub_str_len]
+                if sub_str == sub_str[::-1]:
+                    result += 1
+                start_idx += 1
+
+            sub_str_len += 1
+        
+
+        return result
+
+# DP 풀이
+# O(N^2) times, O(N^2) spaces
+# start, end 지점을 순회하면서 이전 계산값을 재사용하여 회문을 파악
+class Solution2:
+    def countSubstrings(self, s: str) -> int:
+        dp = {}
+
+        for end in range(len(s)):
+            for start in range(end, -1, -1):
+                if start == end:
+                    dp[(start, end)] = True
+                elif start + 1 == end:
+                    dp[(start, end)] = s[start] == s[end]
+                else:
+                    dp[(start, end)] = s[start] == s[end] and dp[(start+1, end-1)]
+
+        return list(dp.values()).count(True)

word-search/jinah92.py

@@ -0,0 +1,24 @@
+# O(M*N*4^W) times, O(W) spaces
+class Solution:
+    def exist(self, board: List[List[str]], word: str) -> bool:
+        rows, cols = len(board), len(board[0])
+    
+        directions = [(0, 1), (0, -1), (1, 0), (-1, 0)]
+
+        def dfs(row, col, idx):
+            if idx == len(word):
+                return True
+            if not (0 <= row < rows and 0 <= col < cols):
+                return False
+            if board[row][col] != word[idx]:
+                return False
+            
+            temp_val = board[row][col]
+            board[row][col] = ""
+
+            result = any(dfs(row+r, col+c, idx+1) for (r, c) in directions)
+
+            board[row][col] = temp_val
+            return result
+    
+        return any(dfs(r, c, 0) for r in range(rows) for c in range(cols))