[Evan] Week 3

climbing-stairs/evan.js

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+/**
+ * @param {number} n
+ * @return {number}
+ */
+var climbStairs = function (n) {
+  if (n <= 2) {
+    return n;
+  }
+
+  let [firstStair, secondStair] = [1, 2];
+  let thirdStair;
+
+  for (let i = 3; i <= n; i++) {
+    thirdStair = firstStair + secondStair;
+
+    [firstStair, secondStair] = [secondStair, thirdStair];
+  }
+
+  return thirdStair;
+};
+
+// Time Complexity: O(n)
+// Reason: The function uses a loop that iterates from 3 to n,
+//         which means it runs in linear time with respect to n.
+
+// Space Complexity: O(1)
+// Reason: The function uses a fixed amount of extra space
+//         (a few integer variables: first, second, and third).
+//         It does not use any additional data structures
+//         that grow with the input size n.

maximum-depth-of-binary-tree/evan.js

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+/**
+ * Definition for a binary tree node.
+ * 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 {number}
+ */
+var maxDepth = function (root) {
+  if (root === null) {
+    return 0;
+  }
+
+  const leftDepth = maxDepth(root.left);
+  const rightDepth = maxDepth(root.right);
+
+  return Math.max(leftDepth, rightDepth) + 1;
+};
+
+/**
+ * Time Complexity: O(n), where n is the number of nodes in the binary tree.
+ * Reason:
+ *  the function visits each node exactly once in order to compute the depth of the tree,
+ *  which ensures that each node is processed a single time.
+ *
+ * Space Complexity: O(h), where h is the height of the binary tree.
+ * Reason:
+ *   The recursion stack used during the depth-first traversal.
+ *
+ *   In the worst case, where the tree is completely unbalanced,
+ *   the height h can be equal to the number of nodes n, leading to a space complexity of O(n).
+ *
+ *   In the best case, where the tree is balanced, the height h is log(n),
+ *   leading to a space complexity of O(log(n)).
+ */

meeting-rooms/evan.js

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+/**
+ * Definition of Interval:
+ * class Interval {
+ *   constructor(start, end) {
+ *     this.start = start;
+ *     this.end = end;
+ *   }
+ * }
+ */
+
+export class Solution {
+  /**
+   * @param intervals: an array of meeting time intervals
+   * @return: if a person could attend all meetings
+   */
+  canAttendMeetings(intervals) {
+    intervals.sort((a, b) => a[0] - b[0]);
+
+    for (let i = 1; i < intervals.length; i++) {
+      const startTime = intervals[i][0];
+      const previousMeetingEndTime = intervals[i - 1][1];
+
+      if (previousMeetingEndTime > startTime) {
+        return false;
+      }
+    }
+
+    return true;
+  }
+}
+
+/**
+ * Time complexity: O(nlogn), where n is the number of meetings
+ * Reason:
+ *  We sort the meetings by start time, which takes O(nlogn) time.
+ *  Then we iterate through the meetings once, which takes O(n) time.
+ *
+ * Space complexity: O(1)
+ * Reason: We use a constant amount of extra space.
+ */

same-tree/evan.js

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+/**
+ * @param {TreeNode} p
+ * @param {TreeNode} q
+ * @return {boolean}
+ */
+var isSameTree = function (p, q) {
+  if (!p && !q) {
+    return true;
+  }
+
+  if (!p || !q || p.val !== q.val) {
+    return false;
+  }
+
+  return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
+};
+
+/**
+ * Time Complexity: O(n)
+ * Reason: We visit each node exactly once.
+ *
+ * Space Complexity: O(n)
+ * Reason:
+ *  The space used by the call stack is proportional
+ *  to the height of the tree.
+ */

subtree-of-another-tree/evan.js

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+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} p
+ * @param {TreeNode} q
+ * @return {boolean}
+ */
+var isSameTree = function (p, q) {
+  if (!p && !q) {
+    return true;
+  }
+
+  if (!p || !q || p.val !== q.val) {
+    return false;
+  }
+
+  return isSameTree(p.left, q.left) && isSameTree(p.right, q.right);
+};
+
+/**
+ * @param {TreeNode} root
+ * @param {TreeNode} subRoot
+ * @return {boolean}
+ */
+var isSubtree = function (root, subRoot) {
+  if (subRoot === null) {
+    return true;
+  }
+
+  if (root === null && subRoot !== null) {
+    return false;
+  }
+
+  if (isSameTree(root, subRoot)) {
+    return true;
+  }
+
+  return isSubtree(root.left, subRoot) || isSubtree(root.right, subRoot);
+};
+
+/**
+ * Time Complexity: O(n * m), n: number of nodes in tree `subRoot`, m: number of nodes in tree `root`
+ * Reason:
+ *   The `isSameTree` function is called at every node of tree `root`.
+ *   The time complexity of the `isSameTree` function is O(n) since it compares all nodes of the two trees,
+ *   where n is the number of nodes in tree `subRoot`.
+ *
+ *   Since `isSameTree` is called at every node of tree `root`,
+ *   the worst-case time complexity is O(n * m),
+ *   where m is the number of nodes in tree `root`.
+ */
+
+/**
+ * Space Complexity: O(h), where h is the height of tree `root`
+ * Reason:
+ *   The space complexity is determined by the depth of the recursion stack.
+ *   In the worst case, the recursion will reach the maximum depth of tree `root`, which is its height h.
+ *   Therefore, the space complexity is O(h).
+ */