come across sth./sbdy.（尤指偶然）遇到，碰上，發(fā)現(xiàn)eg:She came across some old letters in the courseof her search.她在找東西時偶然發(fā)現(xiàn)了一些舊信件。

run across sth./sbdy.與……不期而遇，偶然碰上（尤指令人愉快的人或事物）eg:I ran across an old friend in the street.我在馬路上偶然碰見了一個老朋友。