[saysimple] 2주차 문제 풀이입니다.

invert-binary-tree/saysimple.py

@@ -0,0 +1,20 @@
+# 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
+# TC, SC: O(n), O(n)
+class Solution:
+    def invertTree(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        if not root:
+            return None
+
+        left = root.left
+        root.left = root.right
+        root.right = left
+
+        self.invertTree(root.left)
+        self.invertTree(root.right)
+
+        return root

linked-list-cycle/saysimple.py

@@ -0,0 +1,20 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, x):
+#         self.val = x
+#         self.next = None
+# TC, SC: O(n), O(1)
+class Solution:
+    def hasCycle(self, head: Optional[ListNode]) -> bool:
+        if not head:
+            return None
+
+        head.pos = 0
+
+        while head.next:
+            if hasattr(head.next, "pos"):
+                return True
+            head.next.pos = head.pos + 1
+            head = head.next
+
+        return False

merge-two-sorted-lists/saysimple.py

@@ -0,0 +1,55 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+# TC, SC: O(n), O(n)
+class Solution:
+    def mergeTwoLists(self, list1: Optional[ListNode], list2: Optional[ListNode]) -> Optional[ListNode]:
+        if not (list1 and list2):
+            return list1 if list1 else list2
+
+        ret = ListNode(0, None)
+        head = ret
+
+        while list1 and list2:
+            a, b = list1.val, list2.val
+            print(a, b)
+
+            if a < b:
+                print(1)
+                next = ListNode(a, None)
+                ret.next = next
+                ret = next
+                list1 = list1.next
+            elif a > b:
+                print(2)
+                next = ListNode(b, None)
+                ret.next = next
+                ret = next
+                list2 = list2.next
+            else:
+                for i in range(2):
+                    next = ListNode(a, None)
+                    ret.next = next
+                    ret = next
+                list1, list2 = list1.next, list2.next
+            print(head)
+
+        if list1:
+            while list1:
+                next = ListNode(list1.val, None)
+                ret.next = next
+                ret = next
+                list1 = list1.next
+
+        if list2:
+            while list2:
+                next = ListNode(list2.val, None)
+                ret.next = next
+                ret = next
+                list2 = list2.next
+
+        head = head.next
+
+        return head

reverse-linked-list/saysimple.py

@@ -0,0 +1,27 @@
+# Definition for singly-linked list.
+# class ListNode:
+#     def __init__(self, val=0, next=None):
+#         self.val = val
+#         self.next = next
+# TC, SC: O(n), O(1)
+class Solution:
+    def reverseList(self, head: Optional[ListNode]) -> Optional[ListNode]:
+        if not head:
+            return None
+
+        if not head.next:
+            return head
+
+        prev_node = head
+        node = prev_node.next
+        prev_node.next = None
+
+        while node.next:
+            next_node = node.next
+            node.next = prev_node
+            prev_node = node
+            node = next_node
+
+        node.next = prev_node
+
+        return node

valid-parentheses/saysimple.py

@@ -0,0 +1,24 @@
+# TC, SC: O(n), O(n)
+class Solution:
+    def isValid(self, s: str) -> bool:
+        arr = []
+
+        for c in s:
+            if c in ["(", "[", "{"]:
+                arr.append(c)
+                continue
+            else:
+                if not arr:
+                    return False
+                b = arr.pop()
+                if b == "(" and c == ")":
+                    continue
+                if b == "[" and c == "]":
+                    continue
+                if b == "{" and c == "}":
+                    continue
+                return False
+        if arr:
+            return False
+        else:
+            return True