79. Word Search （Matrix）

运用dfs查找
用substring不太好 可能弄到空的 还是用start记录位置比较好

dfs刚开始先判断条件 判断完再干别的会比较方便

visited 每次用完要变回false 否则有同一个点搜索路径退回的时候，那边就不能被继续搜索了

 

 

class Solution {
    static boolean[][] visited;
    public boolean exist(char[][] board, String word) {
        int n = word.length();
        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)) {
                    if(dfs(i, j, board, 0, word)) {
                        return true;
                    }
                }
            }
        }
        return false;
        
    }
    
    public boolean dfs(int i, int j, char[][] board, int start, String word) {
        if(word.length() == start ) return true;
        if(i < 0 || i >= board.length || j<0 || j >= board[0].length || board[i][j] != word.charAt(start) || visited[i][j] ) {
            return false;
        }
        visited[i][j] = true;
        start++;
        boolean res = dfs(i+1, j, board, start, word) || dfs(i-1, j, board, start, word)
                      ||dfs(i, j-1, board, start, word)
                      ||dfs(i, j+1, board, start, word);
        visited[i][j] = false; //必要的！！
        return res;
        
    }
}