LeetCode-in-All

79. Word Search

Medium

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.

Example 1:

Input: board = [[“A”,”B”,”C”,”E”],[“S”,”F”,”C”,”S”],[“A”,”D”,”E”,”E”]], word = “ABCCED”

Output: true

Example 2:

Input: board = [[“A”,”B”,”C”,”E”],[“S”,”F”,”C”,”S”],[“A”,”D”,”E”,”E”]], word = “SEE”

Output: true

Example 3:

Input: board = [[“A”,”B”,”C”,”E”],[“S”,”F”,”C”,”S”],[“A”,”D”,”E”,”E”]], word = “ABCB”

Output: false

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.

Follow up: Could you use search pruning to make your solution faster with a larger board?

Solution

class Solution {
    /**
     * @param String[][] $board
     * @param String $word
     * @return Boolean
     */
    public function exist($board, $word) {
        $wordCountMap = [];
        for ($i = 0; $i < strlen($word); $i++) {
            $char = $word[$i];
            if (isset($wordCountMap[$char])) {
                $wordCountMap[$char] += 1;
            } else {
                $wordCountMap[$char] = 1;
            }
        }
        $positions = [];
        foreach ($board as $rowIdx => $items) {
            foreach ($items as $colIdx => $item) {
                if ($item === $word[0]) {
                    $positions[] = [$rowIdx, $colIdx];
                }
                if (isset($wordCountMap[$item])) {
                    $wordCountMap[$item]--;
                }
            }
        }
        foreach ($wordCountMap as $count) {
            if ($count > 0) {
                return false;
            }
        }
        foreach ($positions as $position) {
            if ($this->dfs($board, $word, 0, $position)) {
                return true;
            }
        }
        return false;
    }

    function dfs($board, $word, $index, $position) {
        [$x, $y] = $position;
        if (!isset($board[$x][$y])) {
            return false;
        }
        if ($board[$x][$y] !== $word[$index]) {
            return false;
        } elseif ($index === strlen($word) - 1) {
            return true;
        }
        $board[$x][$y] = '-1';
        $dirs = [[0, 1], [0, -1], [1, 0], [-1, 0]];
        foreach ($dirs as $dir) {
            $newX = $x + $dir[0];
            $newY = $y + $dir[1];
            if ($this->dfs($board, $word, $index + 1, [$newX, $newY])) {
                return true;
            }
        }
        return false;
    }
}

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