79. Word Search

Medium
简直是教科书般的DFS+Backtracking. 几个地方可能会出错：

• 如何开始搜索， 这里是看第一个单词符合就开始搜索，如果返回true就说明找到了一个，找到了就返回true
• 找到一个，要么就是或 || 找到了返回true, 要么就是for循环一个一个试(permutation)，这里是或.这里的backtracking其实就是visited[i][j]最后变回false;

class Solution {
    public boolean exist(char[][] board, String word) {
        if (board == null || board.length == 0 || board[0].length == 0){
            return false;
        }
        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 (board[i][j] == word.charAt(0) && dfsHelper(board, i, j, word, 0, visited)){
                    return true;
                }
            }
        }  
        return false;
    }

    private boolean dfsHelper(char[][] board, int i, int j, String word, int charIndex, boolean[][] visited){
        if (charIndex == word.length()){
            return true;
        }
        if (i < 0 || i >= board.length || j < 0 || j >= board[0].length || visited[i][j] 
            || word.charAt(charIndex) != board[i][j]){
            return false;
        }
        visited[i][j] = true;
        if (dfsHelper(board, i + 1, j, word, charIndex + 1, visited) || dfsHelper(board, i - 1, j, word, charIndex + 1, visited)        || dfsHelper(board, i, j + 1, word, charIndex + 1, visited) || dfsHelper(board, i, j - 1, word, charIndex + 1, visited)){
            return true;
        }   
        visited[i][j] = false;
        return false;
    }
}