一个栈即可，存数字、字符串和[，读到]开始出栈，到[止，再往前就是重复次数
3[z]2[2[y]pq1[2[jk]e1[f]]]ef
3 [ z ] ==> zzz
_ ^
zzz 2 [ 2 [ y ] ==> zzz 2 [ yy
_ ^
zzz 2 [ yy p q 1 [ 2 [ j k ] ==> zzz 2 [ yy p q 1 [ jkjk
___ ^
zzz 2 [ yy p q 1 [ jkjk e 1 [ f ] ==> zzz 2 [ yy p q 1 [ jkjk e f
_ ^
zzz 2 [ yy p q 1 [ jkjk e f ] ==> zzz 2 [ yy p q jkjkef
________ ^
zzz 2 [ yy p q jkjkef ] ==> zzzyypqjkjkefjkjkefjkjkefjkjkef
_____________ ^
zzzyypqjkjkefjkjkefjkjkefjkjkef ef ===> zzzyypqjkjkefjkjkefjkjkefjkjkefef