The answer
Because strings are immutable, adding to a string inside a loop forces the computer to reallocate memory and copy the data repeatedly, resulting in O(N^2) time complexity.
Iterating Through Strings Effectively
Why is string concatenation inside a loop bad?
More FAQs in Iterating Through Strings Effectively
Structurally it is the same. The main difference is that strings are often immutable, meaning you cannot change individual characters in-place during the loop.
Immutability means that once a string is created in memory, its contents cannot be altered. Any modification results in the creation of a brand new string.
Use a mutable data structure like a StringBuilder in Java, or push characters to an array in JS/Python and join them at the end.
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