[EGON] Week12 Solutions

merge-intervals/EGON.py

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+from typing import List
+from unittest import TestCase, main
+
+
+class Solution:
+    def merge(self, intervals: List[List[int]]) -> List[List[int]]:
+        return self.solve_stack(intervals)
+
+    """
+    Runtime: 8 ms (Beats 49.77%)
+    Time Complexity: 
+        - intervals의 길이를 n이라 하면, intervals를  시작 지점을 기준으로 정렬하는데 O(n log n)
+        - intervals를 조회하면서 연산하는데, 내부 연산은 모두 O(1)이므로 O(n)
+        > O(n log n) + O(n) ~= O(n log n)
+
+    Memory: 19.88 MB (Beats: 99.31%)
+    Space Complexity: O(n)
+        - intervals는 내부 정렬만 했으므로 추가 메모리 사용 메모리 없음
+        - 겹치는 구간이 없는 최악의 경우 merged의 크기는 intervals의 크기와 같아지므로, O(n) upper bound
+    """
+
+    def solve_stack(self, intervals: List[List[int]]) -> List[List[int]]:
+        intervals.sort()
+
+        merged = []
+        for interval in intervals:
+            if not merged:
+                merged.append(interval)
+                continue
+
+            new_start, new_end = interval
+            _, last_end = merged[-1]
+            if last_end < new_start:
+                merged.append(interval)
+            else:
+                merged[-1][1] = max(last_end, new_end)
+
+        return merged
+
+
+class _LeetCodeTestCases(TestCase):
+
+    def test_1(self):
+        intervals = [[1, 3], [2, 6], [8, 10], [15, 18]]
+        output = [[1, 6], [8, 10], [15, 18]]
+
+        self.assertEqual(Solution().merge(intervals), output)
+
+    def test_2(self):
+        intervals = [[1, 4], [4, 5]]
+        output = [[1, 5]]
+
+        self.assertEqual(Solution().merge(intervals), output)
+
+
+if __name__ == '__main__':
+    main()

number-of-connected-components-in-an-undirected-graph/EGON.py

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+from typing import List
+from unittest import TestCase, main
+
+
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+
+class Solution:
+    def countComponents(self, n: int, edges: List[List[int]]) -> int:
+        return self.solve_union_find(n, edges)
+
+    """
+    LintCode 로그인이 안되어서 https://neetcode.io/problems/count-connected-components에서 실행시키고 통과만 확인했습니다.
+
+    Runtime: ? ms (Beats ?%)
+    Time Complexity: O(max(m, n))
+        - UnionFind의 parent 생성에 O(n)
+        - edges 조회에 O(m)
+            - Union-find 알고리즘의 union을 매 조회마다 사용하므로, * O(α(n)) (α는 아커만 함수의 역함수)
+        - UnionFind의 각 노드의 부모를 찾기 위해, n번 find에 O(n * α(n))
+        > O(n) + O(m * α(n)) + O(n * α(n)) ~= O(max(m, n) *  α(n)) ~= O(max(m, n)) (∵ α(n) ~= C)
+
+    Memory: ? MB (Beats ?%)
+    Space Complexity: O(n)
+        - UnionFind의 parent와 rank가 크기가 n인 리스트이므로, O(n) + O(n)
+        > O(n) + O(n) ~= O(n)
+    """
+
+    def solve_union_find(self, n: int, edges: List[List[int]]) -> int:
+
+        class UnionFind:
+            def __init__(self, size: int):
+                self.parent = [i for i in range(size)]
+                self.rank = [1] * size
+
+            def union(self, first: int, second: int):
+                first_parent, second_parent = self.find(first), self.find(second)
+
+                if self.rank[first_parent] > self.rank[second_parent]:
+                    self.parent[second_parent] = first_parent
+                elif self.rank[first_parent] < self.rank[second_parent]:
+                    self.parent[first_parent] = second_parent
+                else:
+                    self.parent[second_parent] = first_parent
+                    self.rank[first_parent] += 1
+
+            def find(self, node: int):
+                if self.parent[node] != node:
+                    self.parent[node] = self.find(self.parent[node])
+
+                return self.parent[node]
+
+        union_find = UnionFind(size=n)
+        for first, second in edges:
+            union_find.union(first, second)
+
+        result = set()
+        for i in range(n):
+            result.add(union_find.find(i))
+
+        return len(result)
+
+
+class _LeetCodeTestCases(TestCase):
+    def test_1(self):
+        n = 5
+        edges = [[0, 1], [1, 2], [2, 3], [1, 3], [1, 4]]
+        output = False
+
+        self.assertEqual(Solution().countComponents(n, edges), output)
+
+
+if __name__ == '__main__':
+    main()

remove-nth-node-from-end-of-list/EGON.py

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+from typing import Optional
+from unittest import TestCase, main
+
+
+# Definition for singly-linked list.
+class ListNode:
+    def __init__(self, val=0, next=None):
+        self.val = val
+        self.next = next
+
+    def values(self) -> [int]:
+        result = []
+        node = self
+        while node:
+            result.append(node.val)
+            node = node.next
+
+        return result
+
+
+class Solution:
+    def removeNthFromEnd(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        return self.solve_two_pointer(head, n)
+    """
+    Runtime: 0 ms (Beats 100.00%)
+    Time Complexity: O(n)
+    > list의 모든 node + dummy node를 탐색하므로 O(n + 1) ~= O(n)
+
+    Memory: 16.62 MB (Beats 15.78%)
+    Space Complexity: O(1)
+    > 포인터만 사용하므로 O(1)
+    """
+    def solve_two_pointer(self, head: Optional[ListNode], n: int) -> Optional[ListNode]:
+        if head.next is None:
+            return None
+
+        dummy = ListNode(-1)
+        dummy.next = head
+        head = dummy
+
+        slow = fast = head
+        while n:
+            if fast.next:
+                fast = fast.next
+            n -= 1
+
+        while fast is not None:
+            if fast.next is None:
+                slow.next = slow.next.next
+                break
+            else:
+                slow = slow.next
+                fast = fast.next
+
+        return head.next
+
+
+class _LeetCodeTestCases(TestCase):
+    def test_1(self):
+        node_1 = ListNode(1)
+        node_2 = ListNode(2)
+        node_3 = ListNode(3)
+        node_4 = ListNode(4)
+        node_5 = ListNode(5)
+
+        node_1.next = node_2
+        node_2.next = node_3
+        node_3.next = node_4
+        node_4.next = node_5
+
+        n = 2
+        output = [1, 2, 3, 5]
+        self.assertEqual(Solution().removeNthFromEnd(node_1, n).values(), output)
+
+    def test_2(self):
+        node_1 = ListNode(1)
+        n = 1
+        output = []
+        self.assertEqual(Solution().removeNthFromEnd(node_1, n).values(), output)
+
+    def test_3(self):
+        node_1 = ListNode(1)
+        node_2 = ListNode(2)
+        node_1.next = node_2
+        n = 2
+        output = [2]
+        self.assertEqual(Solution().removeNthFromEnd(node_1, n).values(), output)
+
+
+if __name__ == '__main__':
+    main()

same-tree/EGON.py

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+from typing import Optional
+from unittest import TestCase, main
+
+
+# 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 isSameTree(self, p: Optional[TreeNode], q: Optional[TreeNode]) -> bool:
+        return self.solve_dfs(p, q)
+
+    """
+    Runtime: 0 ms (Beats 100.00%)
+    Time Complexity: O(min(p, q))
+    > dfs를 통해 모든 node를 방문하므로, 각 트리의 node의 갯수를 각각 p, q라 하면, O(min(p, q))
+
+    Memory: 16.62 MB (Beats 15.78%)
+    Space Complexity: O(min(p, q))
+    > 일반적인 경우 트리의 깊이만큼 dfs 호출 스택이 쌓이나, 최악의 경우 한쪽으로 편향되었다면 O(min(p, q)), upper bound
+    """
+    def solve_dfs(self, p: Optional[TreeNode], q: Optional[TreeNode]) -> bool:
+
+        def dfs(p: Optional[TreeNode], q: Optional[TreeNode]) -> bool:
+            if p is None and q is None:
+                return True
+            elif (p is not None and q is not None) and (p.val == q.val):
+                return dfs(p.left, q.left) and dfs(p.right, q.right)
+            else:
+                return False
+
+        return dfs(p, q)
+
+
+class _LeetCodeTestCases(TestCase):
+    def test_1(self):
+        p_1 = TreeNode(1)
+        p_2 = TreeNode(2)
+        p_3 = TreeNode(3)
+        p_1.left = p_2
+        p_1.right = p_3
+
+        q_1 = TreeNode(1)
+        q_2 = TreeNode(3)
+        q_3 = TreeNode(3)
+        q_1.left = q_2
+        q_1.right = q_3
+
+        self.assertEqual(Solution().isSameTree(p_1, q_1), True)
+
+
+if __name__ == '__main__':
+    main()

serialize-and-deserialize-binary-tree/EGON.py

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+from typing import Optional
+
+
+# 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 Codec:
+    """
+    Runtime: 76 ms (Beats 85.72%)
+    Time Complexity: O(n)
+    > dfs를 통해 모든 node를 방문하므로, O(n)
+
+    Memory: 21.40 MB (Beats 12.82%)
+    Space Complexity: O(n)
+    > 일반적인 경우 트리의 깊이만큼 dfs 호출 스택이 쌓이나, 최악의 경우 한쪽으로 편향되었다면 트리의 깊이가 n이 될 수 있으므로 O(n), upper bound
+    """
+
+    def serialize(self, root):
+        """Encodes a tree to a single string.
+
+        :type root: TreeNode
+        :rtype: str
+        """
+
+        def dfs(node: Optional[TreeNode]) -> None:
+            if node is None:
+                result.append("null")
+                return
+            result.append(str(node.val))
+            dfs(node.left)
+            dfs(node.right)
+
+        result = []
+        dfs(root)
+        return ','.join(result)
+
+    def deserialize(self, data):
+        """Decodes your encoded data to tree.
+
+        :type data: str
+        :rtype: TreeNode
+        """
+
+        def dfs() -> Optional[TreeNode]:
+            val = next(values)
+            if val == "null":
+                return None
+            node = TreeNode(int(val))
+            node.left = dfs()
+            node.right = dfs()
+            return node
+
+        values = iter(data.split(','))
+        return dfs()
+
+
+# Your Codec object will be instantiated and called as such:
+# ser = Codec()
+# deser = Codec()
+# ans = deser.deserialize(ser.serialize(root))