Need coding problems to practice STL algorithms

I am creating a list of coding problems which I can provide to my juniors to get habitual to using STL algorithm rather writing their own algorithm.

If you have any intuitive coding problem in mind where we can use the STL algorithm efficiently, you will be a great help. Combination of more than one algorithm is also good.

For example: Trim spaces from following string.

int main()
{
    string input_string = "1         2  3   4 5 55 5     ";

    // Your code

    return 0;
}

Output

12345555

Here we can apply std::unique, perfectly.