剑指 Offer 12. 矩阵中的路径

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和求岛屿的数量那类型题一样，都是利用了dfs和回溯

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在回溯的时候，将board[i][j]沉没后，记得在dfs后要恢复

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;
        char temp=board[i][j];
        board[i][j]='?';
        boolean res=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]=temp;//回溯必须要恢复
        return res;
    }
}