s1与s1合并之后的字符串//英语翻译

s1与s1合并之后的字符串//英语翻译
s1与s1合并之后的字符串//英语翻译

s1与s1合并之后的字符串//英语翻译
the string (that is) built/constructed by/after the combination of s1 and s1

a (new) string after merging s2 into s1

the string merged from s1 and a1