[Tessa1217] Week 11 Solutions

binary-tree-maximum-path-sum/Tessa1217.java

@@ -0,0 +1,42 @@
+/**
+ * Definition for a binary tree node.
+ * public class TreeNode {
+ *     int val;
+ *     TreeNode left;
+ *     TreeNode right;
+ *     TreeNode() {}
+ *     TreeNode(int val) { this.val = val; }
+ *     TreeNode(int val, TreeNode left, TreeNode right) {
+ *         this.val = val;
+ *         this.left = left;
+ *         this.right = right;
+ *     }
+ * }
+ */
+class Solution {
+
+    // 최대 Path 누적 합
+    int maxPathSumVal = Integer.MIN_VALUE;
+
+    // 시간복잡도: O(n), 공간복잡도: O(h) h as height of tree
+    public int maxPathSum(TreeNode root) {
+        maxPathSumChecker(root);
+        return maxPathSumVal;    
+    }
+
+    // 재귀
+    private int maxPathSumChecker(TreeNode node) {
+
+        if (node == null) {
+            return 0;
+        }
+
+        int leftMax = Math.max(maxPathSumChecker(node.left), 0);
+        int rightMax = Math.max(maxPathSumChecker(node.right), 0);
+
+        maxPathSumVal = Math.max(node.val + leftMax + rightMax, maxPathSumVal);
+
+        return node.val + Math.max(leftMax, rightMax);
+    }
+}
+

graph-valid-tree/Tessa1217.java

@@ -0,0 +1,57 @@
+public class Solution {
+    /**
+     * @param n: An integer
+     * @param edges: a list of undirected edges
+     * @return: true if it's a valid tree, or false
+     */
+    // 시간 복잡도: O(n), 공간복잡도: O(n)
+    public boolean validTree(int n, int[][] edges) {
+
+        if (edges.length != (n - 1)) {
+            return false; 
+        }
+
+        List<Integer>[] graph = new ArrayList[n];
+        for (int i = 0; i < n; i++) {
+            graph[i] = new ArrayList<>();
+        }
+
+        for (int[] edge : edges) {
+            graph[edge[0]].add(edge[1]);
+            graph[edge[1]].add(edge[0]);
+        }
+
+        boolean[] visited = new boolean[n];
+
+        if (!dfs(0, -1, visited, graph)) {
+            return false;
+        }
+
+        for (boolean v : visited) {
+            if (!v) {
+                return false; // Not fully connected
+            }
+        }
+
+        return true;
+    }
+
+    private boolean dfs(int node, int parent, boolean[] visited, List<Integer>[] graph) {
+        if (visited[node]) {
+            return false; // Found a cycle
+        }
+
+        visited[node] = true;
+
+        for (int neighbor : graph[node]) {
+            if (neighbor != parent) {
+                if (!dfs(neighbor, node, visited, graph)) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+}
+

merge-intervals/Tessa1217.java

@@ -0,0 +1,37 @@
+class Solution {
+    public int[][] merge(int[][] intervals) {
+
+        if (intervals.length <= 1) {
+            return intervals;
+        }
+
+        List<int[]> answer = new ArrayList<>();
+
+        Arrays.sort(intervals, (a, b) -> Integer.compare(a[0], b[0]));
+
+        int start = intervals[0][0];
+        int end = intervals[0][1];
+
+        for (int i = 1; i < intervals.length; i++) {
+
+            // 현재 인터벌 시작점
+            int currStart = intervals[i][0];
+            // 현재 인터벌 끝점 
+            int currEnd = intervals[i][1];
+
+            if (end >= currStart) {
+                end = Math.max(end, currEnd);                
+            } else {
+                answer.add(new int[]{start, end});
+                start = currStart;      
+                end = currEnd;
+            }            
+        }
+
+        answer.add(new int[]{start, end});
+
+        return answer.stream()
+                            .toArray(int[][]::new);
+    }
+}
+

missing-number/Tessa1217.java

@@ -0,0 +1,24 @@
+/**
+ * 0 ~ n까지 고유한 숫자들로 이루어진 배열 nums가 주어질 때 해당 범위 내에서 없어진 숫자를 찾으세요.
+ */
+class Solution {
+
+    // 시간복잡도: O(n), 공간복잡도: O(n)
+    public int missingNumber(int[] nums) {
+
+        boolean[] visitNum = new boolean[nums.length + 1];
+
+        for (int i = 0; i < nums.length; i++) {
+            visitNum[nums[i]] = true;
+        }
+
+        for (int i = 0; i < visitNum.length; i++) {
+            if (!visitNum[i]) {
+                return i;
+            }
+        }
+
+        return 0;
+    }
+}
+

reorder-list/Tessa1217.java

@@ -0,0 +1,59 @@
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    // 시간복잡도: O(n), 공간복잡도: O(1)
+    public void reorderList(ListNode head) {
+
+        // Slow, Fast Pointer
+        // 1 - 2 - 3 - 4 - 5
+        ListNode slow = head;
+        ListNode fast = head;
+
+        // fast가 끝까지 닿을 때 중점 찾기
+        // slow = 3
+        while (fast != null && fast.next != null) {
+            slow = slow.next;
+            fast = fast.next.next;
+        }
+
+
+        // 중간 이후부터 끝까지 (second half of list) => reverse order
+        // 3 이후부터 -> 4 - 5
+        // reverse - 5 - 4
+        ListNode backHead = null; // second half of list's new head
+        ListNode backSide = slow.next;
+        slow.next = null;
+
+        while (backSide != null) {
+            ListNode temp = backSide.next;
+            backSide.next = backHead;
+            backHead = backSide;
+            backSide = temp;
+        }
+
+        // Merge first and second half
+
+        // 1 - 2 - 3    
+        ListNode firstHalf = head;
+        // 5 - 4
+        ListNode secondHalf = backHead;
+
+        // 1 - 5 - 2 - 4 - 3
+        while (secondHalf != null) {
+            ListNode temp = firstHalf.next;
+            firstHalf.next = secondHalf;
+            firstHalf = secondHalf;
+            secondHalf = temp;
+        }
+    }
+}
+
+