[LC] 79. Word Search

Given a 2D board and a word, find if the word exists in the grid.

The word can be constructed from letters of sequentially adjacent cell, where "adjacent" cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once.

Example:

board =
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]

Given word = "ABCCED", return true.
Given word = "SEE", return true.
Given word = "ABCB", return false.

Time: O(M * N * 4^|Word|)

class Solution {
    int row;
    int col;
    public boolean exist(char[][] board, String word) {
        row = board.length;
        col = board[0].length;
        boolean[][] visited = new boolean[row][col];
        for (int i = 0; i < row; i++) {
            for (int j = 0; j < col; j++) {
                if (helper(board, visited, word, i, j, 0)) {
                    return true;
                }
            }
        }
        return false;
    }
    
    private boolean helper(char[][] board, boolean[][] visited, String word, int i, int j, int index) {
        if (index == word.length()) {
            return true;
        }
        if (i < 0 || i >= row || j < 0 || j >= col) {
            return false;
        }
        if (word.charAt(index) == board[i][j] && !visited[i][j]) {
            visited[i][j] = true;
            boolean res = helper(board, visited, word, i + 1, j, index + 1) ||
                helper(board, visited, word, i - 1, j, index + 1) ||
                helper(board, visited, word, i, j + 1, index + 1) ||
                helper(board, visited, word, i, j - 1, index + 1);
            // need to clean up visited
            visited[i][j] = false;
            return res;
        }

        return false;
    }
}

class Solution {
    int[][] directions = new int[][]{{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
    public boolean exist(char[][] board, String word) {
        boolean[][] visited = new boolean[board.length][board[0].length];
        for (int i = 0; i < board.length; i++) {
            for (int j = 0; j < board[0].length; j++) {
                if (dfs(0, word, board, visited, i, j)) {
                    return true;   
                }
            }
        }
        return false;
    }
    
    private boolean dfs(int index, String word, char[][] board, boolean[][] visited, int row, int col) {
        if (index == word.length()) {
            return true;
        }
        // need to check edge after base case e.g. [['A']] 'A', is actually out of board
        if (row < 0 || row >= board.length || col < 0 || col >= board[0].length || visited[row][col]) {
                return false;
        }
        if (board[row][col] == word.charAt(index)) {
            visited[row][col] = true;
            for (int[] direction: directions) {
                int nxtRow = direction[0] + row;
                int nxtCol = direction[1] + col;
                if (dfs(index + 1, word, board, visited, nxtRow, nxtCol)) {
                    return true;
                }
            }
            visited[row][col] = false;
        }
        return false;
    }
}