[daehyun99] WEEK02 solutions

3sum/daehyun99.py

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+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        res = []
+        nums.sort()
+
+        for i, a in enumerate(nums):
+            if a > 0:
+                break
+            if i > 0 and a == nums[i-1]:
+                continue
+
+            l, r = i+1, len(nums) - 1
+            while l < r:
+                total = a + nums[l] + nums[r]
+                if total > 0:
+                    r -= 1
+                elif total < 0:
+                    l += 1
+                else:
+                    res.append([a, nums[l], nums[r]])
+                    l += 1
+                    r -= 1
+                    while nums[l] == nums[l - 1] and l < r:
+                        l += 1
+        return res
+
+"""from collections import Counter
+
+class Solution:
+    def threeSum(self, nums: list[int]) -> list[list[int]]:
+        counts = Counter(nums)
+        result: set[tuple[int, int, int]] = set()
+
+        for i in range(len(nums) - 2):
+            for j in range(i + 1, len(nums) - 1):
+                adding = -(nums[i] + nums[j])
+
+                if adding not in counts:
+                    continue
+
+                local_counts = Counter([nums[i], nums[j], adding])
+
+                for num, count in local_counts.items():
+                    if counts[num] < count:
+                        break
+                else:
+                    triplet = tuple(sorted([nums[i], nums[j], adding]))
+                    result.add(triplet)
+
+        return [list(triplet) for triplet in result]
+"""

climbing-stairs/daehyun99.py

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+class Solution:
+    def climbStairs(self, n: int) -> int:
+        one_count = n
+        two_count = 0
+        total_count = 0
+
+        while one_count >=0 and two_count >=0:
+            # 조합
+            total = one_count + two_count
+
+            count = 1
+            for i in range(two_count):
+                count *= total - i
+            for i in range(two_count, 0, -1):
+                count /= i
+            total_count += count
+
+            one_count -=2
+            two_count +=1
+        return int(total_count)

product-of-array-except-self/daehyun99.py

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+class Solution:
+    def productExceptSelf(self, nums: List[int]) -> List[int]:
+        product = 1
+        zero_pos = set()
+
+        for i in range(len(nums)):
+            num = nums[i]
+            if num != 0:
+                product *= num
+            else:
+                zero_pos.add(i)
+
+        if len(zero_pos) >= 2:
+            return [0 for num in nums]
+        elif len(zero_pos) == 1:
+            return [0 if i not in zero_pos else product for i in range(len(nums))]
+        else:
+            return [int(product / num) for num in nums]

valid-anagram/daehyun99.py

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+from collections import defaultdict
+class Solution:
+    def isAnagram(self, s: str, t: str) -> bool:
+        count = defaultdict(int)
+
+        if len(s) != len(t):
+            return False
+
+        for s_, t_ in zip(s, t):
+            count[s_] += 1
+            count[t_] -= 1
+
+        for key, val in count.items():
+            if val != 0:
+                return False
+        return True

validate-binary-search-tree/daehyun99.py

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+# 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 isValidBST(self, root: Optional[TreeNode]) -> bool:
+        def valid(node, left, right):
+            if not node:
+                return True
+            if not (left < node.val < right):
+                return False
+
+            return (
+                valid(node.left, left, node.val) and
+                valid(node.right, node.val, right)
+            )
+        return valid(root, float("-inf"), float("inf"))