Problem Statement

Given an m x n grid of characters board and a string word, return true if the 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.

Example 1:

• Input: word="ABCCED", board:

    { 'A', 'B', 'C', 'E' },
    { 'S', 'F', 'C', 'S' },
    { 'A', 'D', 'E', 'E' }
  

• Output: true

• Explanation: The word exists in the board:
  -> { 'A', 'B', 'C', 'E' },
  -> { 'S', 'F', 'C', 'S' },
  -> { 'A', 'D', 'E', 'E' }

Example 2:

• Input: word="SEE", board:

    { 'A', 'B', 'C', 'E' },
    { 'S', 'F', 'C', 'S' },
    { 'A', 'D', 'E', 'E' }
  

• Output: true

• Explanation: The word exists in the board:
  -> { 'A', 'B', 'C', 'E' },
  -> { 'S', 'F', 'C', 'S' },
  -> { 'A', 'D', 'E', 'E' }

Constraints:

• m == board.length
• n = board[i].length
• 1 <= m, n <= 6
• 1 <= word.length <= 15
• board and word consists of only lowercase and uppercase English letters.

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