Decode String with `(group){k}` Repeat Syntax

Phone screen variant of LeetCode 394 (Decode String). The repeat-count syntax is `(group){k}` instead of `k[group]`. Nested repeats such as `"a(b(c){2}){2}"` must expand correctly. Solvable with the same stack pattern as the canonical problem. Requirements Input: a string composed of literal characters, parenthesized groups, and {k} suffixes that repeat the preceding group k times. Nesting is allowed: "a(b(c){2}){2}" expands to "abccbcc". Output: the fully expanded decoded string. Example: "abs(cs){3}g" → "abscscscsg". Notes The same two patterns that solve LC 394 work here: an explicit stack (pushing partial strings and their pending repeat counts), or a recursive descent that returns the decoded suffix at each ) boundary. The lexer is the only meaningful difference from the canonical prompt: the repeat count appears after the closing paren in {k} form rather than before the opening bracket. Pre-scan / tokenize before applying the stack walk to keep the control flow clean. Edge cases worth surfacing: k = 0 (drop the group), missing {...} after ) (treat as multiplier of 1), multi-digit k, characters between groups ((a){2}b(c){3} → "aabccc"). Complexity is O(N + M) where N is input length and M is output length; reject the temptation to repeatedly slice the partial string inside the loop (that drives the complexity to O(M²)). Preparation Drill LC 394 (Decode String) until the stack pattern is automatic, then rewrite it for the (group){k} syntax in under 20 minutes. Walk through the nested example by hand on paper before coding — the off-by-one between 'where does the group end' and 'where does {k} end' is the most common bug. Have a 3-line tokenizer ready that emits (, ), {int}, and literal runs so the parser can ignore lexing details.