Leetcode-79: word search

subject

Topic linking
Given a m x n Two dimensional character grid board And a string word word . If word Exists in the grid , return true ; otherwise , return false .

The words must be in alphabetical order , It's made up of letters in adjacent cells , among “ adjacent ” Cells are those adjacent horizontally or vertically . Letters in the same cell are not allowed to be reused .

Example 1：

 Input ：board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCCED"
 Output ：true

Example 2：

 Input ：board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "SEE"
 Output ：true

Example 3：

 Input ：board = [["A","B","C","E"],["S","F","C","S"],["A","D","E","E"]], word = "ABCB"
 Output ：false

Problem solving

Method 1 ： to flash back

class Solution {
    
public:
    vector<vector<int>> dirs={
    {
    1,0},{
    -1,0},{
    0,1},{
    0,-1}};
    vector<vector<bool>> visit;
    bool dfs(vector<vector<char>>& board,string& word,int index,int x,int y){
    
        if(index==word.size()) return true;
        if(x<0||x>=board.size()||y<0||y>=board[0].size()) return false;
        if(visit[x][y]) return false;
        char c=word[index];
        if(board[x][y]!=c) return false;
        for(vector<int>& dir:dirs){
    
            int nx=x+dir[0];
            int ny=y+dir[1];
            visit[x][y]=true;
            if(dfs(board,word,index+1,nx,ny)) return true;
            visit[x][y]=false;
        }
        return false;

    }
    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++){
    
                visit=vector<vector<bool>>(board.size(),vector<bool>(board[0].size(),false));
                if(dfs(board,word,0,i,j)) return true;
            }
        }
        return false;
    }
};