说明：  求子字符串，如字符串为hart，则子字符串会显示为h,ha,har,hart,hat,hr,hrt,ht,a,ar,art,r,rt,t。显示是有规则的，从a开始，是去掉ha的h。
(Child-giving strings, such as the string for the hart, the sub-string will be shown as h, ha, har, hart, hat, hr, hrt, ht, a, ar, art, r, rt, t. Show that there is a rule, from a starting point for the removal of ha of h.)