Input data is given as a string, e.g., '1 2 3 4 3 2 1'. Write an optimal solution in terms of complexity.

If the input data is a string of numbers, for example '1 2 3 4 3 2 1', and you need to perform the task optimally in terms of time, then usually it is advisable to:

1. Split the string into individual numbers.
2. Process them in a single pass using an appropriate algorithm (for example, counting frequencies, finding the maximum, etc.).

Here's an example in Python that counts the frequency of each number in O(n):

input_str = '1 2 3 4 3 2 1'
numbers = map(int, input_str.split())
frequency = {}
for num in numbers:
    frequency[num] = frequency.get(num, 0) + 1
print(frequency)

This approach is linear in time and optimal for most tasks involving processing a sequence of numbers.