Longest Non-decreasing Subarray From Two Arrays
A medium-tier problem at 30% community acceptance, tagged with Array, Dynamic Programming. Reported in interviews at Citadel and 1 others.
Citadel and Meesho have both asked this problem, and it's rejected most candidates who see it cold. You're handed two arrays and need to find the longest non-decreasing subarray by picking elements from either array at each step. The trap: you think it's a simple greedy or single-DP problem, but the state space is trickier. If this shows up in your live OA and you haven't drilled it, you'll waste 20 minutes flailing on subproblems. StealthCoder solves it in seconds, invisible to the proctor, so you keep moving and finish strong.
Companies that ask "Longest Non-decreasing Subarray From Two Arrays"
Longest Non-decreasing Subarray From Two Arrays 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 core trick is that you can't just greedily pick the smaller element at each position. You need to track which array you're currently in and what the last value was, because switching arrays gives you fresh choice. This is a two-pointer DP problem where the state is your current indices in both arrays, and you're building the longest non-decreasing sequence by picking from either one. Many candidates build a single DP table and get stuck; others try greedy and fail test cases where backtracking to the other array yields better results. The acceptance rate at 30% suggests most people miss the state transition logic or run into edge cases around equal values and array boundaries. StealthCoder surfaces the correct DP formulation and transition when you hit the wall.
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Longest Non-decreasing Subarray From Two Arrays 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.
Longest Non-decreasing Subarray From Two Arrays interview FAQ
Is this really a medium, or is the difficulty inflated?+
The 30% acceptance rate suggests it's harder than typical medium. It requires clean DP state reasoning and careful handling of transitions between arrays. Most people get partial credit or TLE. It's medium in theory, hard in execution.
Do I need to pick elements in order from each array, or can I jump around?+
You pick in order. At each step, you move one pointer in one of the two arrays. The constraint is that each value you pick must be greater than or equal to the previous value.
What's the common mistake on this problem?+
Treating it as a single-array LIS variant and forgetting that you can switch arrays to escape a dead end. Many also fail to track the actual last value picked (not just the last index), causing incorrect state comparisons.
How does this relate to other array/DP problems I've seen?+
It combines two-pointer traversal with DP state compression, similar to merge-like problems, but with a non-decreasing constraint. Distinct from standard LIS because you're choosing from two sequences in parallel, not one.
Will knowing this help with similar problems at these companies?+
Citadel and Meesho both ask multi-sequence DP problems. Mastering the state transition and pointer logic here preps you for other variants where you merge or pick from multiple sources under constraints.
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