From each pointer, spread to the board. One tricky is set the char that used to be 0. Set it back when finished searching.

1 class Solution { 2 private: 3     bool found; 4     int n, m; 5 public: 6     void findWord(vector
     
      
        > &board, int index, int row, int column, string word) { 7         if (word[index] != board[row][column] || found) { 8             return; 9         }10         if (index == word.size()-1) {11             found = true;12             return;13         }14         char tmp = board[row][column];15         board[row][column] = 0;16         if (row > 0) findWord(board, index+1, row-1, column, word);17         if (row < n-1) findWord(board, index+1, row+1, column, word);18         if (column > 0) findWord(board, index+1, row, column-1, word);19         if (column < m-1) findWord(board, index+1, row, column+1, word);20         board[row][column] = tmp;21     }22     bool exist(vector
       
        
          > &board, string word) {23         if (board.size() == 0) return false;24         n = board.size(), m = board[0].size();25         found = false;26         for (int i = 0; i < n; i++) {27             for (int j = 0; j < m; j++) {28                 findWord(board, 0, i, j, word);29                 if (found) return true;30             }31         }32         return false;33     }34 };