Decode String

To solve the Decode String problem efficiently, I would implement an iterative solution using stacks to handle the nested repetition logic. First, I initialize two stacks: one for storing the repeat counts (integers) and another for the partial strings constructed so far. As I iterate through the input string character by character, I check the type of each character. If it is a digit, I build the full number by multiplying the current result by ten and adding the new digit. When I encounter an opening bracket '[', I know a new context begins. I push the accumulated number onto the count stack and the current string fragment onto the string stack, then reset both for the new segment. Upon hitting a closing bracket ']', I pop the most recent count and the previous string context. I then pop the current segment, repeat it the specified number of times, and append it to the popped previous string. This effectively merges the nested layers back up. Finally, if there are remaining characters after the loop, I append them to the result. For example, processing '3[a2[c]]' would correctly yield 'accaccacc'. This approach ensures we handle arbitrary nesting depths in linear time relative to the output size, which aligns with Apple's focus on clean, efficient, and robust system design principles.