就一个起点，没必要回溯

    1.     int movingCount(int threshold, int rows, int cols)
    2.     {
    3.         if(!rows || !cols)return 0;
    4.         vector<vector<bool>> visited(rows, vector<bool>(cols, false));
    5.         return backtrack(threshold, rows, cols, 0, 0, visited);
    6.     }
    7.     int backtrack(int threshold, int rows, int cols, int r, int c, vector<vector<bool>> & visited)
    8.     {   
    9.         int count = 0;
    10.         if(canEnter(threshold, r, c) && r >= 0 && r < rows && c >= 0 && c < cols && !visited[r][c]){
    11.             visited[r][c] = true;
    12.             count = 1 + backtrack(threshold, rows, cols, r + 1, c, visited) + backtrack(threshold, rows, cols, r, c + 1, visited) + backtrack(threshold, rows, cols, r - 1, c, visited) + backtrack(threshold, rows, cols, r, c - 1, visited);
    13.         }
    14.         return count;
    15.     }
    16.     bool canEnter(int threshold, int r, int c){
    17.         int cnt = 0;
    18.         while(r != 0){
    19.             cnt += r % 10;
    20.             r = r / 10;
    21.         }
    22.         while(c != 0){
    23.             cnt += c % 10;
    24.             c = c / 10;
    25.         }
    26.         if(cnt <= threshold)return true;
    27.         else return false;
    28.     }