Construct the Lexicographically Largest Valid Sequence
A medium-tier problem at 73% community acceptance, tagged with Array, Backtracking. Reported in interviews at Salesforce and 0 others.
You're staring at a sequence construction problem that looks simple until you realize the greedy approach doesn't work. Construct the Lexicographically Largest Valid Sequence is the kind of medium-tier problem that trips up candidates who skip the backtracking angle. Salesforce has asked this one. The trap is obvious: just pick the largest number available at each step, right. Wrong. You need to think backwards, understand spacing constraints, and use backtracking to prune bad branches fast. If this hits your live assessment and you blank on the structural insight, StealthCoder surfaces the working solution in seconds, invisible to the proctor.
Companies that ask "Construct the Lexicographically Largest Valid Sequence"
Construct the Lexicographically Largest Valid Sequence 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. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround.
Get StealthCoderThe core trick is realizing you can't simply greedy-select large numbers without violating the spacing rule between duplicate values. You're building a sequence where each number i from n down to 1 appears either once or twice, and if it appears twice, the two occurrences must be exactly i positions apart. The lexicographically largest sequence means you want bigger numbers earlier. Backtracking with pruning is your tool: try to place the largest available number at each position, check if the remaining slots can still be filled validly, and backtrack when you hit a dead end. The acceptance rate sits above 70 percent, but that includes people who got lucky with a partial solution or tried every permutation brute-force. The Array and Backtracking topics tell you the interviewer wants to see both constraint satisfaction and search efficiency. StealthCoder hedges the case where you see the problem for the first time and need a working solution fast.
Pattern tags
You know the problem.
Make sure you actually pass it.
Construct the Lexicographically Largest Valid Sequence 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. Built because the OA filter rejects engineers who'd pass the on-site. That's a broken filter. This is the workaround. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Construct the Lexicographically Largest Valid Sequence interview FAQ
Is this just a greedy problem where I pick the biggest number at each spot?+
No. Picking the largest number greedily doesn't guarantee you can fill the rest of the sequence. You need backtracking to explore choices, validate spacing constraints, and backtrack when a path fails. The trick is trying large numbers first (for lexicographic order) but verifying feasibility before committing.
What's the core constraint I need to model?+
If number i appears twice in the sequence, the two positions must be exactly i steps apart. If it appears once, there's no spacing rule. This constraint is what makes backtracking necessary: you can't place a duplicate i without reserving a spot i positions away.
How hard is this really compared to other medium backtracking problems?+
Medium difficulty is fair. The acceptance rate above 70 percent suggests it's solvable by most candidates who recognize the backtracking pattern. The trick isn't algorithmic complexity but modeling the constraint correctly and pruning early to avoid timeout.
Does Salesforce ask this in their actual OA, or is it historical?+
Salesforce has reported this problem. It's the kind of constraint-satisfaction, backtracking problem that fits their interview style. Prepare it seriously if you're interviewing there, but don't assume it will appear, treat it as one of many possible mediums.
What's my approach to building a solution under time pressure?+
Sketch the constraint first: duplicates need i-spacing. Then code a backtracking helper that tries filling positions from left to right, attempts largest-to-smallest values at each spot, and backtracks if you can't feasibly fill the rest. Test on small examples before submitting.
Want the actual problem statement? View "Construct the Lexicographically Largest Valid Sequence" on LeetCode →