[Helena] Week 7 solutions

binary-tree-level-order-traversal/yolophg.js

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+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+var levelOrder = function (root) {
+  // if the root is null, return an empty array.
+  if (root === null) return [];
+
+  const result = [];
+  const queue = [root];
+
+  // while there are nodes in the queue,
+  while (queue.length > 0) {
+    const levelSize = queue.length;
+    const currentLevel = [];
+
+    // loop nodes in the current level.
+    for (let i = 0; i < levelSize; i++) {
+      // dequeue the front node.
+      const currentNode = queue.shift();
+      // add value to the current level array.
+      currentLevel.push(currentNode.val);
+      // enqueue left child if exists.
+      if (currentNode.left) queue.push(currentNode.left);
+      // enqueue right child if exists.
+      if (currentNode.right) queue.push(currentNode.right);
+    }
+
+    // add the current level array to the result.
+    result.push(currentLevel);
+  }
+
+  return result;
+};

lowest-common-ancestor-of-a-binary-search-tree/yolophg.js

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+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+var lowestCommonAncestor = function (root, p, q) {
+  // start from the root.
+  let current = root;
+
+  // traverse the tree.
+  while (current !== null) {
+    // if both p and q are greater than current node, LCA lies in the right.
+    if (p.val > current.val && q.val > current.val) {
+      current = current.right;
+    }
+    // if both p and q are smaller than current node, LCA lies in the left.
+    else if (p.val < current.val && q.val < current.val) {
+      current = current.left;
+    }
+    // if one of p or q is on one side and the other is on the other side, It's LCA.
+    else {
+      return current;
+    }
+  }
+
+  // if the tree is empty.
+  return null;
+};

remove-nth-node-from-end-of-list/yolophg.js

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+// Time Complexity: O(n)
+// Space Complexity: O(1)
+
+var removeNthFromEnd = function (head, n) {
+  // calculate the length of the linked list.
+  let length = 0;
+  let current = head;
+  while (current !== null) {
+    length++;
+    current = current.next;
+  }
+
+  // determine the position to remove from the start.
+  let removeIndex = length - n;
+
+  // if the node to be removed is the head, return the next node.
+  if (removeIndex === 0) {
+    return head.next;
+  }
+
+  // traverse to the node just before the node to be removed.
+  current = head;
+  for (let i = 0; i < removeIndex - 1; i++) {
+    current = current.next;
+  }
+
+  // remove the nth node from the end.
+  current.next = current.next.next;
+
+  // return the modified list.
+  return head;
+};

reorder-list/yolophg.js

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+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+var reorderList = function (head) {
+  // push all nodes onto a stack.
+  let stack = [];
+  let current = head;
+  while (current) {
+    stack.push(current);
+    current = current.next;
+  }
+
+  // reorder the list.
+  let n = stack.length;
+  current = head;
+
+  for (let i = 0; i < Math.floor(n / 2); i++) {
+    let next = current.next;
+    let last = stack.pop();
+
+    current.next = last;
+    last.next = next;
+    current = next;
+  }
+  // ensure the last node points to null.
+  if (current) current.next = null;
+};

validate-binary-search-tree/yolophg.js

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+// Time Complexity: O(n)
+// Space Complexity: O(n)
+
+var isValidBST = function (root) {
+  if (root === null) {
+    return true;
+  }
+
+  // initialize a queue for BFS.
+  let queue = [];
+  queue.push({ node: root, min: -Infinity, max: Infinity });
+
+  while (queue.length > 0) {
+    // dequeue the front one.
+    let { node, min, max } = queue.shift();
+
+    // check the BST for the current node.
+    if (node.val <= min || node.val >= max) {
+      return false;
+    }
+
+    // enqueue the left child with updated min and max.
+    if (node.left !== null) {
+      queue.push({ node: node.left, min: min, max: node.val });
+    }
+
+    // enqueue the right child with updated min and max.
+    if (node.right !== null) {
+      queue.push({ node: node.right, min: node.val, max: max });
+    }
+  }
+
+  return true;
+};