week2 mission done

Author: dev-jonghoonpark

invert-binary-tree/dev-jonghoonpark.md

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+- https://leetcode.com/problems/invert-binary-tree/
+- time complexity : O(n)
+- space complexity : O(log n)
+- https://algorithm.jonghoonpark.com/2024/03/31/leetcode-226
+
+```java
+public class Solution {
+    public TreeNode invertTree(TreeNode root) {
+        if (root == null) {
+            return null;
+        }
+
+        var temp = root.left;
+        root.left = root.right;
+        root.right = temp;
+
+        if (root.left != null) {
+            invertTree(root.left);
+        }
+        if (root.right != null) {
+            invertTree(root.right);
+        }
+
+        return root;
+    }
+}
+```

linked-list-cycle/dev-jonghoonpark.md

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+- https://leetcode.com/problems/linked-list-cycle
+- time complexity : O(n)
+- space complexity : O(1)
+- https://algorithm.jonghoonpark.com/2024/02/15/leetcode-141
+
+```java
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        if (head == null) {
+            return false;
+        }
+
+        ListNode p1 = head;
+        ListNode p2 = head;
+        while(p2 != null && p2.next != null) {
+            p1 = p1.next;
+            p2 = p2.next.next;
+
+            if (p1 == p2) {
+                return true;
+            }
+        }
+
+        return false;
+    }
+}
+```

merge-two-sorted-lists/dev-jonghoonpark.md

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+- https://leetcode.com/problems/merge-two-sorted-lists/
+- time complexity : O(n)
+- space complexity : O(n)
+- https://algorithm.jonghoonpark.com/2024/05/01/leetcode-21
+
+```java
+class Solution {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        ListNode head = null;
+        ListNode tail = null;
+
+        while(!(list1 == null && list2 == null)) {
+            ListNode selected;
+            if (list1 == null) {
+                selected = list2;
+                list2 = list2.next;
+            } else if (list2 == null) {
+                selected = list1;
+                list1 = list1.next;
+            } else if (list1.val < list2.val) {
+                selected = list1;
+                list1 = list1.next;
+            } else {
+                selected = list2;
+                list2 = list2.next;
+            }
+
+            ListNode newNode = new ListNode(selected.val);
+            if(head == null) {
+                head = newNode;
+            } else {
+                tail.next = newNode;
+            }
+
+            tail = newNode;
+        }
+
+        return head;
+    }
+}
+```

reverse-linked-list/dev-jonghoonpark.md

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+- https://leetcode.com/problems/reverse-linked-list/
+- time complexity : O(n)
+- space complexity : O(1)
+- https://algorithm.jonghoonpark.com/2024/02/15/leetcode-206
+
+```java
+class Solution {
+    public ListNode reverseList(ListNode head) {
+        if(head == null) {
+            return null;
+        }
+
+        ListNode p1 = head;
+        ListNode p2 = head.next;
+        p1.next = null;
+
+        while (p2 != null) {
+            ListNode temp = p2.next;
+            p2.next = p1;
+            p1 = p2;
+            p2 = temp;
+        }
+
+        return p1;
+    }
+}
+```

valid-parentheses/dev-jonghoonpark.md

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+- https://leetcode.com/problems/valid-parentheses/
+- time complexity : O(n)
+- space complexity : O(n)
+- https://algorithm.jonghoonpark.com/2024/04/29/leetcode-20
+
+```java
+class Solution {
+    public boolean isValid(String s) {
+        Stack<Character> stack = new Stack<>();
+
+        for (char c : s.toCharArray()) {
+            if (c == '(' || c == '{' || c == '[') {
+                stack.push(c);
+                continue;
+            }
+
+            if (stack.isEmpty()) {
+                return false;
+            }
+
+            boolean valid = false;
+            if (c == ')') {
+                valid = stack.peek() == '(';
+            } else if (c == '}') {
+                valid = stack.peek() == '{';
+            } else if (c == ']') {
+                valid = stack.peek() == '[';
+            }
+
+            if (valid) {
+                stack.pop();
+            } else {
+                return false;
+            }
+        }
+
+        return stack.isEmpty();
+    }
+}
+```