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[coloryourlife] Week2 solutions #364

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Aug 24, 2024
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Original file line number Diff line number Diff line change
@@ -0,0 +1,23 @@
# 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:
# T: O(N)
# S: O(N)
def buildTree(self, preorder: List[int], inorder: List[int]) -> Optional[TreeNode]:
# preorder : root - left - right
# inorder : left - root - right
if not preorder and not inorder:
return None

root = TreeNode(preorder[0])
mid = inorder.index(preorder[0])

root.left = self.buildTree(preorder[1 : mid + 1], inorder[:mid])
root.right = self.buildTree(preorder[mid + 1 :], inorder[mid+1:])

return root

15 changes: 15 additions & 0 deletions counting-bits/coloryourlife.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,15 @@
# T: O(N)
# S: O(N)
class Solution:
def countBits(self, n: int) -> List[int]:
res = []
for i in range(n + 1):
m = i
cnt = 0
while m > 0:
if m & 1 == 1:
cnt += 1
m = m >> 1
res.append(cnt)
return res

23 changes: 23 additions & 0 deletions decode-ways/coloryourlife.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,23 @@
class Solution:
# T: O(N)
# S: O(N)
def numDecodings(self, s: str) -> int:
# The subproblem would be any portion of string
dp = {len(s): 1}

def dfs(i):
if i in dp:
return dp[i]
if s[i] == "0":
return 0

res = dfs(i + 1)
if i + 1 < len(s) and (
s[i] == "1" or s[i] == "2" and s[i + 1] in "0123456"
):
res += dfs(i + 2)
dp[i] = res
return res

return dfs(0)

25 changes: 25 additions & 0 deletions encode-and-decode-strings/coloryourlife.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,25 @@
class Codec:
# T: O(n)
# S: O(1)
def encode(self, strs: List[str]) -> str:
"""Encodes a list of strings to a single string.
"""
res = ""
for s in strs:
res += str(len(s)) + "#" + s
return res


def decode(self, s: str) -> List[str]:
"""Decodes a single string to a list of strings.
"""
res, i = [], 0
while i < len(s):
j = i
while s[j] != "#":
j += 1
length = int(s[i:j])
res.append(s[j + 1 : j + 1 + length])
i = j + 1 + length
return res

18 changes: 18 additions & 0 deletions valid-anagram/coloryourlife.py
Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
class Solution:
def isAnagram(self, s: str, t: str) -> bool:
# 1. sorting and compare : O(nlogn)
# 2. hashing: O(n)
# 3. array: O(n)
if len(s) != len(t):
return False
char_dict = defaultdict(int)
for c in s:
char_dict[c] += 1

for c in t:
char_dict[c] -= 1
if char_dict[c] < 0:
return False

return True

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