[Wan] Week 4

merge-two-sorted-lists/taewanseoul.ts

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+/**
+ * 21. Merge Two Sorted Lists
+ * You are given the heads of two sorted linked lists list1 and list2.
+ * Merge the two lists into one sorted list. The list should be made by splicing together the nodes of the first two lists.
+ * Return the head of the merged linked list.
+ *
+ * https://leetcode.com/problems/merge-two-sorted-lists/description/
+ */
+
+/**
+ * Definition for singly-linked list.
+ * 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)
+ *     }
+ * }
+ */
+
+// O(n + m) time
+// O(n + m) space
+function mergeTwoLists(
+  list1: ListNode | null,
+  list2: ListNode | null
+): ListNode | null {
+  if (!list1) return list2;
+  if (!list2) return list1;
+
+  let mergedListNode: ListNode;
+  const val1 = list1.val;
+  const val2 = list2.val;
+  if (val1 > val2) {
+    mergedListNode = new ListNode(val2);
+    mergedListNode.next = mergeTwoLists(list1, list2.next);
+  } else {
+    mergedListNode = new ListNode(val1);
+    mergedListNode.next = mergeTwoLists(list1.next, list2);
+  }
+
+  return mergedListNode;
+}
+
+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;
+  }
+}

missing-number/taewanseoul.ts

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+/**
+ * 268. Missing Number
+ * Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.
+ *
+ * https://leetcode.com/problems/missing-number/description/
+ */
+
+// O(n) time
+// O(1) space
+function missingNumber(nums: number[]): number {
+  const n = nums.length;
+  let total = (n * (n + 1)) / 2;
+
+  let sum = 0;
+  for (let i = 0; i < n; i++) {
+    sum += nums[i];
+  }
+
+  return total - sum;
+}

word-search/taewanseoul.ts

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+/**
+ * 268. Missing Number
+ * Given an m x n grid of characters board and a string word, return true if word exists in the grid.
+ * The word can be constructed from letters of sequentially adjacent cells, where adjacent cells are horizontally or vertically neighboring. The same letter cell may not be used more than once.
+ *
+ * https://leetcode.com/problems/word-search/description/
+ */
+
+// O(4^n) time
+// O(n * m) space
+function exist(board: string[][], word: string): boolean {
+  let result = false;
+  const num_rows = board.length;
+  const num_cols = board[0].length;
+
+  function checkNeighbors(
+    board: (string | null)[][],
+    word: string,
+    row: number,
+    col: number,
+    startIndex: number
+  ) {
+    const num_rows = board.length;
+    const num_cols = board[0].length;
+
+    if (row < 0 || row >= num_rows || col < 0 || col >= num_cols) return;
+
+    if (board[row][col] !== word[startIndex]) return;
+
+    if (startIndex === word.length - 1) {
+      result = true;
+      return;
+    }
+
+    board[row][col] = null;
+    checkNeighbors(board, word, row + 1, col, startIndex + 1);
+    checkNeighbors(board, word, row - 1, col, startIndex + 1);
+    checkNeighbors(board, word, row, col + 1, startIndex + 1);
+    checkNeighbors(board, word, row, col - 1, startIndex + 1);
+    board[row][col] = word[startIndex];
+  }
+
+  for (let i = 0; i < num_rows; i++) {
+    for (let j = 0; j < num_cols; j++) {
+      if (board[i][j] === word[0]) {
+        checkNeighbors(board, word, i, j, 0);
+        if (result) return result;
+      }
+    }
+  }
+
+  return result;
+}