The answer
Because they create O(N^2) quadratic time complexity, which scales terribly. As data grows, execution time explodes, causing performance bottlenecks.
Why You Should Avoid Nested Loops When Possible
Why are nested loops considered bad practice?
More FAQs in Why You Should Avoid Nested Loops When Possible
Yes, you should verbally explain it or quickly write it to establish a working brute-force baseline, but you must immediately follow up by optimizing it.
A Hash Map stores previous loop data in memory with O(1) instant lookup. Instead of an inner loop searching for a value, the code just asks the Hash Map if it exists.
Yes, for intrinsically multi-dimensional tasks, like traversing a 2D matrix or processing an image grid, nested loops are strictly required and optimal.
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