Maximum Length of a Concatenated String with Unique Characters
A medium-tier problem at 54% community acceptance, tagged with Array, String, Backtracking. Reported in interviews at Palo Alto Networks and 0 others.
Maximum Length of a Concatenated String with Unique Characters is a medium-difficulty problem that trips up candidates who overthink the constraint. You're given an array of strings and need to concatenate subsets of them such that the result has no duplicate characters, then return the longest possible length. Palo Alto Networks asks it. The naive greedy approach fails because you can't just pick the longest strings first. You need backtracking to explore all valid combinations, and the key insight is that bit manipulation can make character tracking fast and clean.
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Maximum Length of a Concatenated String with Unique Characters is the kind of problem that decides whether you pass. StealthCoder reads the problem on screen and surfaces a working solution in under 2 seconds. Invisible to screen share. The proctor sees nothing. Made for the engineer who has done the work but might still blank with a webcam pointed at him.
Get StealthCoderThe trick is recognizing this is a subset-selection problem, not a string-matching one. Backtracking lets you try every possible combination of strings, checking if adding each new string maintains the uniqueness constraint. Most candidates jump to greedy or dynamic programming and hit a wall. The pattern: for each string, decide to include it or skip it. Use a bitmask or set to track which characters are already used, and prune branches where adding a string would introduce duplicates. If you haven't drilled the backtracking template for this specific constraint shape, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
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Maximum Length of a Concatenated String with Unique Characters recycles across companies for a reason. It's medium-tier, and most candidates blank under the timer. StealthCoder is the hedge: an AI overlay invisible during screen share. It reads the problem and surfaces a working solution in under 2 seconds. Made for the engineer who has done the work but might still blank with a webcam pointed at him. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Maximum Length of a Concatenated String with Unique Characters interview FAQ
Why doesn't a greedy approach work here?+
Greedy fails because the longest available string might block multiple shorter strings that together would yield a longer result. You have to explore trade-offs between subsets, which is why backtracking is required. Greedy only works if you can guarantee a local choice leads to a global optimum, which isn't true here.
Is bit manipulation necessary or just an optimization?+
It's an optimization. A set-based approach works fine too. Bit manipulation is faster for character collision detection if you're only dealing with lowercase letters (26 bits), but it's not required for correctness. Choose whichever you're faster at during the live assessment.
How do I avoid duplicate work in the recursion?+
You don't need memoization here because the state space is the index in the string array and the current character set. Memoization is tricky because the character set is hard to hash. Instead, just ensure your backtracking doesn't revisit the same (index, charset) pair. For most inputs, the search tree is small enough without it.
What's the time complexity and is it fast enough?+
Worst case is O(2^n) where n is the number of strings, because you explore all subsets. For typical input sizes (15-20 strings), this is acceptable. If you hit a time limit, check if you're pruning branches aggressively. Early termination when a string contains duplicates internally saves a lot of work.
Does this problem relate to other backtracking patterns?+
Yes. It's similar to subset-sum and combination-sum problems but with a string constraint instead of a numeric one. The backtracking skeleton is identical: choose, recurse, undo. If you've practiced those patterns, the structure feels familiar. The novelty is just tracking character uniqueness instead of a running sum.
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