微软面试题： LeetCode 79. 单词搜索 出现次数：2

题目描述：

 代码：

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
    bool exist(vector<vector<char>>& board, string word)
    {
       const int m = board.size();
       const int n = board[0].size();
       dx = {0,1,0,-1};
       dy  = {-1,0,1,0};
       flag.assign(m,vector<int>(n,0));
       word.reserve();
       for(int i = 0;i < m;++i)
       {
           for(int j = 0;j < n;++j)
           {
               if(board[i][j] == word[word.size() - 1])
               {
                   bool exit = dfs(board,i,j,word);
                   if(exit)
                   {
                       return true;
                   }
               }
           }
       }
       return false;
    }

    bool dfs(vector<vector<char>>& board,int i,int j,string &word)
    {
        if(word == "")
        {
            return true;
        }
        if(!(i>=0 && i < board.size() && j >= 0 && j< board[0].size()))
        {
            return false;
        }
        char c = word[word.size() - 1];
        if(flag[i][j] == 1 || board[i][j] != c)
        {
            return false;
        }
        //前进，标记访问过的路径
        flag[i][j] = 1;
        word.pop_back();
        for(int k = 0; k < 4 ;++k)
        {
           bool res = dfs(board,i + dx[k],j + dy[k],word);
           if(res) 
           {
               return true;
           }
        }
        //回退，状态重置
        flag[i][j] = 0;
        word.push_back(c);
        return false;
    }
private:
    vector<vector<int> > flag;
    vector<int> dx;
    vector<int> dy;
};