回溯

回溯可以使用DFS+剪枝解决

class Solution {
    public boolean exist(char[][] board, String word) {
        char[] words=word.toCharArray();
        for(int i=0;i<board.length;i++){
            for(int j=0;j<board[0].length;j++){
                if(dfs(board,words,i,j,0)) return true;
            }
        }
        return false;
    }

    boolean dfs(char[][]board,char[]word,int i,int j,int k){
        if(i>=board.length||i<0||j>=board[0].length||j<0||board[i][j]!=word[k]) return false;
        if(k==word.length-1)return true;
        board[i][j]='\0';
        boolean ret=dfs(board,word,i+1,j,k+1)||dfs(board,word,i-1,j,k+1)||
                    dfs(board,word,i,j+1,k+1)||dfs(board,word,i,j-1,k+1);
        board[i][j]=word[k];
        return ret;
    }
}