Merge pull request #129 from nhistory/week7

[Sehwan] Week7 solution with JavaScript

Author: nhistory

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

@@ -0,0 +1,32 @@
+var levelOrder = function (root) {
+  // Edge case: If root is null, return []
+  if (root === null) return [];
+
+  // Create result and queue
+  let result = [];
+  let queue = [root];
+
+  // Iterate while queue.length is exist
+  while (queue.length) {
+    // Create levelArr and levelSize
+    let levelArr = [];
+    let levelSize = queue.length;
+    // Initiate currentNode from queue by using shift method
+    while (levelSize) {
+      const currentNode = queue.shift();
+      levelArr.push(currentNode.val);
+      // If currentNode.left is not null, push into the queue
+      if (currentNode.left) queue.push(currentNode.left);
+      // If currentNode.right is not null, push into the queue
+      if (currentNode.right) queue.push(currentNode.right);
+
+      levelSize--;
+    }
+    // Push levelArr into result
+    result.push(levelArr);
+  }
+  return result;
+};
+
+// TC: O(n)
+// SC: O(n)

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

@@ -0,0 +1,11 @@
+var lowestCommonAncestor = function (root, p, q) {
+  // Iterate if statement with comparing values
+  while (root) {
+    if (root.val < p.val && root.val < q.val) root = root.right;
+    else if (root.val > p.val && root.val > q.val) root = root.left;
+    else return root;
+  }
+};
+
+// TC: O(n)
+// SC: O(1)

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

@@ -0,0 +1,33 @@
+var removeNthFromEnd = function (head, n) {
+  // Edge case: If the list is empty
+  if (!head) return null;
+
+  // Create a dummy node that points to the head
+  let dummy = new ListNode(0);
+  dummy.next = head;
+  let length = 0,
+    curr = head;
+
+  // Calculate the length of the list
+  while (curr) {
+    length++;
+    curr = curr.next;
+  }
+
+  // Find the length-n node from the beginning
+  length = length - n;
+  curr = dummy;
+  while (length > 0) {
+    length--;
+    curr = curr.next;
+  }
+
+  // Skip the desired node
+  curr.next = curr.next.next;
+
+  // Return the head, which may be a new head if we removed the first node
+  return dummy.next;
+};
+
+// TC: O(n)
+// SC: O(1)

reorder-list/nhistory.js

@@ -0,0 +1,68 @@
+// First option : change node value
+var reorderList = function (head) {
+  if (!head) return;
+  // Create new arr to return result and current head
+  let arr = [];
+  let curr = head;
+  // Make list array that has every values from linked list
+  let list = [];
+  while (curr) {
+    list.push(curr.val);
+    curr = curr.next;
+  }
+  // Iterate list array if i%2 === 0 then curr.val = list[Math.floor(i / 2)]
+  // If i%2 !== 0 them curr.val = list[list.length - Math.floor((i + 1) / 2)]
+  curr = head;
+  for (let i = 0; i < list.length; i++) {
+    if (i % 2 === 0) {
+      curr.val = list[Math.floor(i / 2)];
+    } else {
+      curr.val = list[list.length - Math.floor((i + 1) / 2)];
+    }
+    curr = curr.next;
+  }
+};
+
+// TC: O(n)
+// SC: O(n)
+
+// Second option : move node without changing values
+var reorderList = function (head) {
+  // Edge case
+  if (!head) return;
+
+  // Find mid node from linked list
+  let slow = head,
+    fast = head;
+  while (fast && fast.next) {
+    slow = slow.next;
+    fast = fast.next.next;
+  }
+
+  // Reverse second half list
+  let prev = null,
+    curr = slow,
+    temp;
+  while (curr) {
+    temp = curr.next;
+    curr.next = prev;
+    prev = curr;
+    curr = temp;
+  }
+
+  // Modify linked list as followed instruction
+  let first = head,
+    second = prev;
+  while (second.next) {
+    temp = first.next;
+    first.next = second;
+    first = temp;
+
+    temp = second.next;
+    second.next = first;
+    second = temp;
+  }
+};
+
+// TC: O(n)
+// SC: O(1)

validate-binary-search-tree/nhistory.js

@@ -0,0 +1,13 @@
+var isValidBST = function (root) {
+  return validate(root, -Infinity, Infinity);
+};
+
+function validate(node, min, max) {
+  if (!node) return true; // An empty tree is a valid BST
+  if (node.val <= min || node.val >= max) return false; // Current node's value must be between min and max
+
+  // Recursively validate the left and right subtree
+  return (
+    validate(node.left, min, node.val) && validate(node.right, node.val, max)
+  );
+}