code: function _getCellIndex(table, row, cell) {
          var rowSpanCount = 0;
          for (var i = 0, len = row.cells.length; i < len; i++) {
              if (row.cells[i] == cell) {
                  break;
              }
              rowSpanCount += row.cells[i].rowSpan - 1;
          }
          return cell.cellIndex - rowSpanCount;
      }
docs: 取得下一行cell的index