[bky373] Solutions for week 2 problems

invert-binary-tree/bky373.java

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+/**
+ * - 문제: https://leetcode.com/problems/linked-list-cycle/
+ * - TC: O(N)
+ * - SC: O(N)
+ */
+public class Solution_226 {
+    public TreeNode invertTree(TreeNode root) {
+        if (root == null) {
+            return null;
+        }
+        TreeNode tmp = root.left;
+        root.left = invertTree(root.right);
+        root.right = invertTree(tmp);
+        return root;
+    }
+}

linked-list-cycle/bky373.java

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+/**
+ * - 문제: https://leetcode.com/problems/linked-list-cycle/
+ * - TC: O(N)
+ * - SC: O(1)
+ */
+public class Solution {
+    public boolean hasCycle(ListNode head) {
+        if (head == null || head.next == null) {
+            return false;
+        }
+        int min = -10001;
+
+        while (head.next != null) {
+            if (head.next.val == min) {
+                return true;
+            }
+            head.val = min;
+            head = head.next;
+        }
+        return false;
+    }
+}

merge-two-sorted-lists/bky373.java

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+/**
+ * TC: O(N)
+ * SC: O(N)
+ */
+class Solution_21 {
+    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
+        if (list1 == null && list2 == null) {
+            return null;
+        }
+
+        List<Integer> nums = new ArrayList<>();
+
+        while (list1 != null) {
+            nums.add(list1.val);
+            list1 = list1.next;
+        }
+        while (list2 != null) {
+            nums.add(list2.val);
+            list2 = list2.next;
+        }
+
+        Object[] arr = nums.toArray();
+        Arrays.sort(arr);
+
+        ListNode head = new ListNode((int) arr[0]);
+        ListNode curr = head;
+        for (int i=1; i<arr.length; i++) {
+            curr.next = new ListNode((int) arr[i]);
+            curr = curr.next;
+        }
+        return head;
+    }
+}

reverse-linked-list/bky373.java

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+/**
+ * https://leetcode.com/problems/reverse-linked-list/
+ * TC: O(N)
+ * SC: O(N)
+ */
+class Solution_206 {
+
+    public ListNode reverseList(ListNode head) {
+        if (head == null) {
+            return head;
+        }
+        ListNode a = new ListNode(head.val);
+        ListNode b;
+        while (head.next != null) {
+            b = new ListNode(head.next.val, a);
+            a = b;
+            head = head.next;
+        }
+        return a;
+    }
+}

valid-parentheses/bky373.java

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+/**
+ * - 문제: https://leetcode.com/problems/valid-parentheses/
+ * - TC: O(N)
+ * - SC: O(1)
+ */
+class Solution_20 {
+
+    public boolean isValid(String s) {
+        Stack<Character> st = new Stack<>();
+        Map<Character, Character> pairs = Map.of('(', ')', '{', '}', '[', ']');
+
+        for (int i = 0; i < s.length(); i++) {
+            // 열린 괄호이면 스택에 push
+            if (pairs.containsKey(s.charAt(i))) {
+                st.push(s.charAt(i));
+            } else { // 닫힌 괄호인데
+                if (st.isEmpty()) { // 스택이 비어있으면(열린 괄호가 없으면) false 반환
+                    return false;
+                }
+                // 스택의 최근 요소(열린 괄호)와 짝을 이루지 않으면 false 반환
+                if (s.charAt(i) != pairs.get(st.pop())) {
+                    return false;
+                }
+            }
+        }
+        // 열린 괄호가 남아 있을 수 있으므로 목록이 비어있는지 체크
+        return st.isEmpty();
+    }
+}