Nested Pattern String Expansion

Requirements

• Input is one string inputStr.
• A pattern is a substring enclosed in parentheses, followed by a repeat count enclosed in curly braces.
• Expand each pattern by concatenating it with itself the requested number of times.
• Pattern-count pairs can be nested, such as (ef((ab){2}cd){2}){2}.
• Multi-digit counts can appear.
• Some inputs may contain spaces; skip irrelevant spaces if the statement permits them.

Examples

Input:

(ab){3}

Output:

ababab

Input:

(a(bc){2}){2}

Output:

abcbcabcbc

Notes

• A stack-based parser is the natural fit: keep the string accumulated before (, parse the substring up to ), read the following {number}, then repeat and append.
• Java candidates hit errors when comparing char to String, for example using != "}" instead of comparing against '}'.
• Hidden cases can stress nested groups and multi-digit repeat counts.