From 7728a62aa9533979054598524a55afe7ebdec914 Mon Sep 17 00:00:00 2001
From: LSH <koreas9408@gmail.com>
Date: Sat, 11 May 2024 23:54:06 +0900
Subject: [PATCH] Add week2

---
 invert-binary-tree/Seunghyun-Lim.java     | 20 ++++++++++++
 linked-list-cycle/Seunghyun-Lim.java      | 22 +++++++++++++
 merge-two-sorted-lists/Seunghyun-Lim.java | 14 +++++++++
 reverse-linked-list/Seunghyun-Lim.java    | 26 ++++++++++++++++
 valid-parentheses/Seunghyun-Lim.java      | 38 +++++++++++++++++++++++
 5 files changed, 120 insertions(+)
 create mode 100644 invert-binary-tree/Seunghyun-Lim.java
 create mode 100644 linked-list-cycle/Seunghyun-Lim.java
 create mode 100644 merge-two-sorted-lists/Seunghyun-Lim.java
 create mode 100644 reverse-linked-list/Seunghyun-Lim.java
 create mode 100644 valid-parentheses/Seunghyun-Lim.java

diff --git a/invert-binary-tree/Seunghyun-Lim.java b/invert-binary-tree/Seunghyun-Lim.java
new file mode 100644
index 000000000..6fb9df4e6
--- /dev/null
+++ b/invert-binary-tree/Seunghyun-Lim.java
@@ -0,0 +1,20 @@
+/**
+ * 달레님 강의 참고
+ * 아직 완벽하게 이해하지 못한 풀이,, 추가로 다시 확인 필요
+ * 시간복잡도: O(n)
+ * 공간복잡도: O(n)
+ */
+class Solution {
+    public TreeNode invertTree(TreeNode root) {
+        if (root == null) return null;
+
+        TreeNode temp = root.left;
+        root.left = root.right;
+        root.right = temp;
+
+        invertTree(root.right);
+        invertTree(root.left);
+
+        return root;
+    }
+}
diff --git a/linked-list-cycle/Seunghyun-Lim.java b/linked-list-cycle/Seunghyun-Lim.java
new file mode 100644
index 000000000..b5455151f
--- /dev/null
+++ b/linked-list-cycle/Seunghyun-Lim.java
@@ -0,0 +1,22 @@
+/**
+ * 순환 리스트
+ * 달레님 강의 참고
+ * 기존에 이해했던 Linked List 처럼 null이 되는 경우가 아니기 때문에 loop에 빠지지 않게 주의해야 한다.
+ * 시간복잡도: O(n)
+ * 공간복잡도: O(n)
+ */
+class Solution {
+    public boolean hasCycle(ListNode head) {
+        Set<ListNode> visited = new HashSet<>();
+        while (head != null) {
+            if (visited.contains(head)) {
+                return true;
+            } else {
+                visited.add(head);
+                head = head.next;
+            }
+        }
+
+        return false;
+    }
+}
diff --git a/merge-two-sorted-lists/Seunghyun-Lim.java b/merge-two-sorted-lists/Seunghyun-Lim.java
new file mode 100644
index 000000000..575f34dc5
--- /dev/null
+++ b/merge-two-sorted-lists/Seunghyun-Lim.java
@@ -0,0 +1,14 @@
+class Solution {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        if (list1 == null) return list2;
+        if (list2 == null) return list1;
+
+        if (list1.val < list2.val) {
+            list1.next = mergeTwoLists(list1.next, list2);
+            return list1;
+        } else {
+            list2.next = mergeTwoLists(list1, list2.next);
+            return list2;
+        }
+    }
+}
diff --git a/reverse-linked-list/Seunghyun-Lim.java b/reverse-linked-list/Seunghyun-Lim.java
new file mode 100644
index 000000000..ecfefbf73
--- /dev/null
+++ b/reverse-linked-list/Seunghyun-Lim.java
@@ -0,0 +1,26 @@
+/**
+ * 시간복잡도: O(n)
+ * -> 노드가 존재하지 않을때 까지 반복,,
+ * 공간복잡도: O(1)
+ * -> ListNode
+ */
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        if (head == null || head.next == null) {
+            return head;
+        }
+
+        ListNode node1 = head;
+        ListNode node2 = node1.next;
+        head.next = null;
+
+        while (node1 != null && node2 != null) {
+            ListNode t = node2.next;
+            node2.next = node1;
+            node1 = node2;
+            node2 = t;
+        }
+
+        return node1;
+    }
+}
diff --git a/valid-parentheses/Seunghyun-Lim.java b/valid-parentheses/Seunghyun-Lim.java
new file mode 100644
index 000000000..e3626788b
--- /dev/null
+++ b/valid-parentheses/Seunghyun-Lim.java
@@ -0,0 +1,38 @@
+/**
+ * 시간복잡도: O(n)
+ * -> 문자열 s의 길이에 따라 증가
+ * 공간복잡도: O(n)
+ * -> 문자열 s의 길이에 따라 stack의 공간 증가
+ */
+class Solution {
+    public boolean isValid(String s) {
+        char first = s.charAt(0);
+        if (s.length() == 1 || first == ')' || first == '}' || first == ']') return false;
+
+        Stack<Character> stack = new Stack<>();
+
+        for (int i = 0; i < s.length(); i++) {
+            char c = s.charAt(i);
+
+            if (c == '(' || c == '{' || c == '[') {
+                stack.push(c);
+                continue;
+            }
+
+            if (stack.isEmpty()) return false;
+
+            if (c == ')') {
+                Character pop = stack.pop();
+                if (pop != '(') return false;
+            } else if (c == '}') {
+                Character pop = stack.pop();
+                if (pop != '{') return false;
+            } else if (c == ']') {
+                Character pop = stack.pop();
+                if (pop != '[') return false;
+            }
+        }
+
+        return stack.size() == 0;
+    }
+}