[Evan] Week 2 Solutions

invert-binary-tree/evan.js

@@ -0,0 +1,22 @@
+function TreeNode(val, left, right) {
+  this.val = val === undefined ? 0 : val;
+  this.left = left === undefined ? null : left;
+  this.right = right === undefined ? null : right;
+}
+/**
+ * @param {TreeNode} root
+ * @return {TreeNode}
+ */
+var invertTree = function (root) {
+  if (!root) {
+    return null;
+  }
+
+  const originLeft = root.left;
+  const originRight = root.right;
+
+  root.right = invertTree(originLeft);
+  root.left = invertTree(originRight);
+
+  return root;
+};

linked-list-cycle/evan.js

@@ -0,0 +1,27 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+
+/**
+ * @param {ListNode} head
+ * @return {boolean}
+ */
+var hasCycle = function (head) {
+  let fastPointer = head;
+  let slowPointer = head;
+
+  while (fastPointer !== null && slowPointer !== null) {
+    fastPointer = fastPointer?.next?.next;
+    slowPointer = slowPointer?.next;
+
+    if (fastPointer === slowPointer) {
+      return true;
+    }
+  }
+
+  return false;
+};

merge-two-sorted-lists/evan.js

@@ -0,0 +1,30 @@
+function ListNode(val, next) {
+  this.val = val === undefined ? 0 : val;
+  this.next = next === undefined ? null : next;
+}
+
+/**
+ * @param {ListNode} list1
+ * @param {ListNode} list2
+ * @return {ListNode}
+ */
+var mergeTwoLists = function (list1, list2) {
+  const dummy = new ListNode();
+  let current = dummy;
+
+  while (list1 !== null && list2 !== null) {
+    if (list1.val < list2.val) {
+      current.next = list1;
+      list1 = list1.next;
+    } else {
+      current.next = list2;
+      list2 = list2.next;
+    }
+
+    current = current.next;
+  }
+
+  current.next = list1 !== null ? list1 : list2;
+
+  return dummy.next;
+};

reverse-linked-list/evan.js

@@ -0,0 +1,25 @@
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val, next) {
+ *     this.val = (val===undefined ? 0 : val)
+ *     this.next = (next===undefined ? null : next)
+ * }
+ */
+/**
+ * @param {ListNode} head
+ * @return {ListNode}
+ */
+var reverseList = function (head) {
+  let currentNode = head;
+  let previousNode = null;
+
+  while (currentNode !== null) {
+    const nextNode = currentNode.next;
+
+    currentNode.next = previousNode;
+    previousNode = currentNode;
+    currentNode = nextNode;
+  }
+
+  return previousNode;
+};

valid-parentheses/evan.js

@@ -0,0 +1,26 @@
+/**
+ * @param {string} s
+ * @return {boolean}
+ */
+var isValid = function (s) {
+  if (s.length % 2 !== 0) {
+    return false;
+  }
+
+  const stack = [];
+  const pair = {
+    ")": "(",
+    "}": "{",
+    "]": "[",
+  };
+
+  for (let char of s) {
+    if (pair[char] && stack.pop() !== pair[char]) {
+      return false;
+    }
+
+    stack.push(char);
+  }
+
+  return stack.length === 0;
+};