#261 Search for Word

Given a 2D board of characters and a word, determine if the word exists in the board.

Rules for word existence:

• Word can be constructed from letters of adjacent cells
• Adjacent means horizontally or vertically neighboring
• Same cell cannot be used more than once
• Board contains only English letters (case-sensitive)

Constraints:

• 1 ≤ board dimensions ≤ 5
• 1 ≤ word.length ≤ 10
• Board and word contain only English letters

Examples:

// Example 1:
const board1 = [
  ["A", "B", "C", "D"],
  ["S", "A", "A", "T"],
  ["A", "C", "A", "E"],
];
console.log(exist(board1, "CAT"));
// Output: true
// Explanation: Can form "CAT" using adjacent cells

// Example 2:
const board2 = [
  ["A", "B", "C", "D"],
  ["S", "A", "A", "T"],
  ["A", "C", "A", "E"],
];
console.log(exist(board2, "BAT"));
// Output: false
// Explanation: Cannot form "BAT" using adjacent cells