This solution has a time complexity of O(n^2) in the worst case scenario, where n is the length of the input string.

This is due to the use of the contains() method, which loops through the remaining substring to check if a character appears more than once.

In the worst case scenario, this will cause the loop to be repeated n times, resulting in O(n^2) time complexity.

is it be more better if we use {for}?

yes, i have same opinion, using api to simplify the code but it have poor performance

Kind of embarassing that a O(n^2) is this high up :/

Nice job!

If in a more general case (not restricted to 26 english character and 10 digits), this method's time complexity could be O(n^2), which I think is not good enough...

this solution doesnt work at all, replace ask for char to char rather than string to string.