[sun]WEEK 2 Solution

construct-binary-tree-from-preorder-and-inorder-traversal/sun912.py

@@ -0,0 +1,19 @@
+"""
+TC: O(n)
+SC: O(n)
+"""
+# 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 buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
+        if inorder:
+            idx = inorder.index(preorder.pop(0))
+            node = TreeNode(inorder[idx])
+            node.left = self.buildTree(preorder, inorder[0:idx])
+            node.right = self.buildTree(preorder, inorder[idx+1:])
+
+            return node

counting-bits/sun912.py

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+"""
+TC: O(n)
+SC: O(n)
+"""
+
+
+class Solution:
+    def countBits(self, n: int) -> List[int]:
+        dp = [0] * (n+1)
+        offset = 1
+
+        for i in range(1, n+1):
+            if offset * 2 == i:
+                offset = i
+            dp[i] = 1 + dp[i-offset]
+        return dp
+
+

decode-ways/sun912.py

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+"""
+    TC: O(n)
+    SC: O(n)
+
+    DNF, read solutions by Dale...ㅠㅠ
+"""
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        memo = {len(s): 1}
+
+        def dfs(start):
+            if start in memo:
+                return memo[start]
+
+            if s[start] == "0":
+                return 0
+            if start + 1 < len(s) and int(s[start: start+2]) < 27:
+                memo[start] = dfs(start + 1) + dfs(start + 2)
+            else:
+                memo[start] = dfs(start + 1)
+            return memo[start]
+
+        return dfs(0)

encode-and-decode-strings/sun912.py

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+"""
+TC: O(n)
+
+"""
+class Solution:
+    """
+    @param: strs: a list of strings
+    @return: encodes a list of strings to a single string.
+    """
+    def encode(self, strs):
+        result = ""
+        for str in strs:
+            result += str(len(str)) + "#" + str
+        return result
+
+    """
+    @param: str: A string
+    @return: decodes a single string to a list of strings
+    """
+    def decode(self, str):
+        result = []
+        idx = 0
+        while idx < len(str):
+            temp_idx = idx
+            while str[temp_idx] != "#":
+                temp_idx += 1
+            length = int(str[idx:temp_idx])
+            result.append(str[temp_idx+1:temp_idx+length+1])
+            idx = temp_idx + length + 1
+        return result

valid-anagram/sun912.py

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+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        if len(s) != len(t):
+            return False
+
+        count_s = {}
+        count_t = {}
+
+        for i in range(len(s)):
+            count_s[s[i]] = 1 + count_s.get(s[i], 0)
+            count_t[t[i]] = 1 + count_t.get(t[i], 0)
+
+        for c in count_t:
+            if count_t[c] != count_s.get(c, 0):
+                return False
+        return True