79.Word Search

思路：

• dfs,重点写在注释里面。这道题做了好久，以后不做无意义的尝试，想好了再改，调试。

class Solution {
public:
    bool exist(vector<vector<char>>& board, string word) {
        for(int i = 0; i < board.size(); i++){
            for(int j = 0; j < board[0].size(); j++){
                if(dfs(board,0,i,j,word))
                    return true;
            }
        }
        return false;
    }
    bool dfs(vector<vector<char>>& board,int cur,int i,int j,string word){
        if(i < 0 || j < 0 || i >= board.size() ||j >= board[0].size()) return false;    //应该放在开头，否则下面board[i][j]有可能越界
        if(cur == word.size()-1 && board[i][j] == word[cur]) return true;
        if(board[i][j] == '0') return false;
        if(board[i][j] != word[cur]) return false;
        if(board[i][j] == word[cur]){
            char tmp = board[i][j];
            board[i][j] = '0';
            if(dfs(board,cur+1,i+1,j,word) || dfs(board,cur+1,i-1,j,word) || dfs(board,cur+1,i,j+1,word) || dfs(board,cur+1,i,j-1,word)) return true;   //这里要有返回结果
            board[i][j] = tmp;
            return false;
        }

    }
};