[Jeehay28] Week 10 Solutions

course-schedule/Jeehay28.ts

@@ -0,0 +1,46 @@
+// TC: O(V + E), V = numCourses, E = prerequisites.length
+// SC: O(V + E)
+
+function canFinish(numCourses: number, prerequisites: number[][]): boolean {
+  // 0: [1]
+  // 1: []
+
+  const graph = new Map<number, number[]>();
+
+  // Initialize all courses
+  for (let i = 0; i < numCourses; i++) {
+    graph.set(i, []);
+  }
+
+  // Build the graph
+  for (const [crs, pre] of prerequisites) {
+    graph.get(crs)!.push(pre);
+  }
+
+  const traversing = new Set<number>();
+  const finished = new Set<number>();
+
+  const dfs = (crs: number): boolean => {
+    if (traversing.has(crs)) return false; // cycle detected
+
+    if (finished.has(crs)) return true; // already visited
+
+    traversing.add(crs);
+
+    for (const pre of graph.get(crs)!) {
+      if (!dfs(pre)) return false;
+    }
+
+    traversing.delete(crs);
+    finished.add(crs);
+
+    return true;
+  };
+
+  for (const crs of graph.keys()) {
+    if (!dfs(crs)) return false;
+  }
+
+  return true;
+}
+

invert-binary-tree/Jeehay28.ts

@@ -0,0 +1,62 @@
+class TreeNode {
+  val: number;
+  left: TreeNode | null;
+  right: TreeNode | null;
+  constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.left = left === undefined ? null : left;
+    this.right = right === undefined ? null : right;
+  }
+}
+
+// TC: O(n)
+// SC: O(n)
+function invertTree(root: TreeNode | null): TreeNode | null {
+  if (!root) return null;
+
+  const left = invertTree(root.right);
+  const right = invertTree(root.left);
+
+  root.left = left;
+  root.right = right;
+
+  return root;
+}
+
+
+// TC: O(n)
+// SC: O(n)
+// function invertTree(root: TreeNode | null): TreeNode | null {
+//     if (!root) return null;
+
+//     [root.left, root.right] = [root.right, root.left];
+
+//     invertTree(root.left);
+//     invertTree(root.right);
+
+//     return root;
+
+// }
+
+
+// TC: O(n)
+// SC: O(n)
+// function invertTree(root: TreeNode | null): TreeNode | null {
+//   if (!root) return null;
+
+//   const stack: (TreeNode | null)[] = [root];
+
+//   while (stack.length > 0) {
+//     const node = stack.pop();
+
+//     if (!node) continue;
+
+//     [node.left, node.right] = [node.right, node.left];
+
+//     stack.push(node.left);
+//     stack.push(node.right);
+//   }
+
+//   return root;
+// }
+

jump-game/Jeehay28.ts

@@ -0,0 +1,40 @@
+// TC: O(n)
+// SC: O(1)
+function canJump(nums: number[]): boolean {
+  //   nums = [3, 2, 1, 0, 4]
+  // farthest  3, 3, 3, 3
+  let farthest = 0;
+
+  for (let i = 0; i < nums.length; i++) {
+    if (i > farthest) return false;
+    farthest = Math.max(farthest, nums[i] + i);
+  }
+  return true;
+}
+
+
+// ❌ TC: O(n^2)
+// SC: O(n)
+
+// function canJump(nums: number[]): boolean {
+//   const lastIndex = nums.length - 1;
+//   const visited = new Map<number, boolean>();
+
+//   const canReach = (current: number): boolean => {
+//     if (current === lastIndex) return true;
+
+//     if (visited.has(current)) return visited.get(current) as boolean;
+
+//     for (let i = 1; i <= nums[current]; i++) {
+//       if (current + i <= lastIndex) {
+//         if (canReach(current + i)) return true;
+//       }
+//     }
+
+//     visited.set(current, false);
+
+//     return false;
+//   };
+
+//   return canReach(0);
+// }

merge-k-sorted-lists/Jeehay28.ts

@@ -0,0 +1,37 @@
+class ListNode {
+  val: number;
+  next: ListNode | null;
+  constructor(val?: number, next?: ListNode | null) {
+    this.val = val === undefined ? 0 : val;
+    this.next = next === undefined ? null : next;
+  }
+}
+
+// TC: O(n * log n), n = total number of all nodes combined (across all lists)
+// SC: O(n)
+// TDDO: Implement more optimized solution using Min Heap with TC: O(n log k)
+function mergeKLists(lists: Array<ListNode | null>): ListNode | null {
+  const nodeValues: number[] = [];
+
+  for (let node of lists) {
+    while (node) {
+      nodeValues.push(node.val);
+      node = node.next;
+    }
+  }
+
+  nodeValues.sort((a, b) => a - b);
+
+  if (nodeValues.length === 0) return null;
+
+  const head = new ListNode(nodeValues[0]);
+  let temp = head;
+
+  for (let i = 1; i < nodeValues.length; i++) {
+    temp.next = new ListNode(nodeValues[i]);
+    temp = temp.next;
+  }
+
+  return head;
+}
+

search-in-rotated-sorted-array/Jeehay28.ts

@@ -0,0 +1,69 @@
+// TC: O(log n)
+// SC: O(1)
+function search(nums: number[], target: number): number {
+  let left = 0;
+  let right = nums.length - 1;
+
+  while (left <= right) {
+    const mid = Math.floor((left + right) / 2);
+
+    if (nums[mid] === target) return mid;
+
+    if (nums[left] <= nums[mid]) {
+      // Left half is sorted
+      if (nums[left] <= target && target < nums[mid]) {
+        right = mid - 1;
+      } else {
+        left = mid + 1;
+      }
+    } else {
+      // Right half is sorted
+      if (nums[mid] < target && target <= nums[right]) {
+        left = mid + 1;
+      } else {
+        right = mid - 1;
+      }
+    }
+  }
+
+  return -1;
+}
+
+// TC: O(log n)
+// SC: O(1)
+// function search(nums: number[], target: number): number {
+//   const findPivot = (nums: number[]) => {
+//     let left = 0;
+//     let right = nums.length - 1;
+
+//     while (left <= right) {
+//       const mid = Math.floor((left + right) / 2);
+//       if (mid > 0 && nums[mid - 1] > nums[mid]) return mid;
+//       if (nums[0] <= nums[mid]) {
+//         left = mid + 1;
+//       } else {
+//         right = mid - 1;
+//       }
+//     }
+//     return 0;
+//   };
+
+//   const binarySearch = (left: number, right: number) => {
+//     while (left <= right) {
+//       const mid = Math.floor((left + right) / 2);
+//       if (nums[mid] === target) return mid;
+//       if (nums[mid] < target) {
+//         left = mid + 1;
+//       } else {
+//         right = mid - 1;
+//       }
+//     }
+//     return -1;
+//   };
+
+//   const pivot = findPivot(nums);
+//   return Math.max(
+//     binarySearch(0, pivot - 1),
+//     binarySearch(pivot, nums.length - 1)
+//   );
+// }