Interleaving String
A medium-tier problem at 42% community acceptance, tagged with String, Dynamic Programming. Reported in interviews at thoughtspot and 12 others.
Interleaving String is a medium-tier dynamic programming problem that shows up at Apple, Microsoft, Amazon, Uber, and other tier-1 shops. The premise seems deceptively simple: given three strings, determine if the third is formed by interleaving the first two while preserving their relative order. Most candidates start with recursion and hit exponential time. The real trick is recognizing this as a 2D DP problem where you track position in both input strings simultaneously. With a 42% acceptance rate, it's a solid filter: you either see the pattern quickly or you're debugging for twenty minutes. If this one lands in your live assessment and the greedy intuition fails, StealthCoder surfaces the DP solution invisibly.
Companies that ask "Interleaving String"
Interleaving String 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 naive approach is trying to match characters greedily or recurse without memoization, both of which fail on tricky inputs where you need to backtrack. The insight is building a 2D DP table where dp[i][j] represents whether the first i characters of string one and first j characters of string two can interleave to form the first i+j characters of the result string. At each state, you either consume from string one or string two, provided the next character matches. Space can be optimized to 1D if needed, but the 2D version is clearer under interview pressure. Edge cases trip people up: empty strings, strings of different lengths that make interleaving impossible, and cases where the result length doesn't equal the sum of input lengths. This is textbook Dynamic Programming and String manipulation. If you freeze during the live OA, StealthCoder handles the DP table construction and delivers a working solution in seconds.
Pattern tags
You know the problem.
Make sure you actually pass it.
Interleaving String 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.
Interleaving String interview FAQ
Why doesn't a greedy or recursive brute-force pass?+
Greedy fails because you often can't decide which string to consume from at each step without seeing future characters. Pure recursion explores all possibilities and hits exponential time. DP memoizes overlapping subproblems (different ways to reach the same (i,j) state) and reduces it to O(m*n) where m and n are the lengths of the two input strings.
Is this still asked at big tech companies?+
Yes. Apple, Microsoft, Amazon, and Uber all report asking it. With 42% acceptance, it's a legitimate screening problem, not a rare edge case. It filters for DP fluency and the ability to model state transitions under time pressure.
What's the exact state transition in the DP?+
dp[i][j] is true if (dp[i-1][j] and s1[i-1]==s3[i+j-1]) or (dp[i][j-1] and s2[j-1]==s3[i+j-1]). You're checking: can I reach this position by consuming from string one, or from string two, while the character matches the result string.
How does this relate to other string and DP problems?+
It combines string matching (like pattern matching or subsequence problems) with 2D DP (like Longest Common Subsequence or edit distance). Mastering the state-table model here prepares you for similar grid-based DP on strings at other companies.
What kills candidates during the live assessment?+
Forgetting to check that len(s1)+len(s2)==len(s3) up front. Not initializing the first row and column correctly (empty string cases). And conflating indices: mixing up which string's character to check at each DP transition. These are silent bugs that produce wrong answers on hidden test cases.
Want the actual problem statement? View "Interleaving String" on LeetCode →