[i-mprovising] Week 03 solutions

combination-sum/i-mprovising.py

@@ -0,0 +1,19 @@
+"""
+Time complexity O(c*t)
+Space complexity O(c*t)
+
+Dynamic programming
+"""
+
+class Solution:
+    def combinationSum(self, candidates: List[int], target: int) -> List[List[int]]:
+        # init dp array
+        dp = [[] for _ in range(target+1)] # dp[i] : combinations to sum to i
+        dp[0] = [[]]
+
+        for candidate in candidates:
+            for num in range(candidate, target+1):
+                for comb in dp[num-candidate]:
+                    dp[num].append(comb + [candidate])
+
+        return dp[-1]

decode-ways/i-mprovising.py

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+"""
+Time complexity O(n)
+Space complexity O(1)
+
+Dynamic programming
+"""
+
+
+class Solution:
+    def numDecodings(self, s: str) -> int:
+        if s[0] == '0':
+            return 0
+        n = len(s)
+        if n == 1:
+            return 1
+        tmp = int(s[:2])
+        dp = [1, 1]
+        if 11 <= tmp <= 26:
+            if tmp != 20:
+                dp = [1, 2]
+        elif s[1] == '0':
+            if tmp not in [10, 20]:
+                return 0
+        if n == 2:
+            return dp[-1]
+
+        for i in range(2, n):
+            if s[i] == '0':
+                if s[i-1] in ['1', '2']:
+                    cur = dp[0]
+                else:
+                    return 0
+            else:
+                cur = dp[1]
+                tmp = int(s[i-1:i+1])
+                if (11 <= tmp <= 26) and (tmp != 20):
+                    cur += dp[0]
+            dp = [dp[1], cur]
+
+        return dp[-1]

maximum-subarray/i-mprovising.py

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+"""
+Time complexity O(n)
+Space complexity O(n)
+
+Dynamic programming
+"""
+
+
+class Solution:
+    def maxSubArray(self, nums: List[int]) -> int:
+        n = len(nums)
+        dp = [0 for _ in range(n)]
+        dp[0] = nums[0]
+
+        for i in range(1, n):
+            dp[i] = max(dp[i-1]+nums[i], nums[i])
+
+        return max(dp)

number-of-1-bits/i-mprovising.py

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+"""
+Time complexity O(n)
+Space complexity O(1)
+"""
+
+class Solution:
+    def hammingWeight(self, n: int) -> int:
+        return bin(n).count('1')

valid-palindrome/i-mprovising.py

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+"""
+Time complexity O(n)
+Space complexity O(n)
+"""
+
+class Solution:
+    def isPalindrome(self, s: str) -> bool:
+        # preprocessing
+        string = [c for c in s.lower() if c.isalnum()]
+
+        if string == [c for c in string[::-1]]:
+            return True
+        return False