Smallest Substring With Identical Characters II
A hard-tier problem at 41% community acceptance, tagged with String, Binary Search. Reported in interviews at Salesforce and 0 others.
Salesforce asks this string problem, and you'll either see the pattern immediately or you'll stare at your screen for five minutes while the timer burns. Smallest Substring With Identical Characters II sits in that hard zone where the naive greedy approach fails silently. You need to find the shortest substring where you can reduce characters to a single character within k operations, where each operation decreases a character by one. The 41% acceptance rate tells you most candidates miss the trick. If this one hits your live assessment and you blank on the binary search angle, StealthCoder surfaces a working solution in seconds, invisible to the proctor.
Companies that ask "Smallest Substring With Identical Characters II"
Smallest Substring With Identical Characters II 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 by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE.
Get StealthCoderThe core trick is binary search on substring length, not greedy iteration. You pick a candidate length, then check if any substring of that length can be reduced to a single character within k operations. The check itself requires finding which character to reduce to (the one with the highest frequency in that window minimizes total operations), then validating the cost. Most people try to build the answer left-to-right, which collapses because the optimal substring doesn't always start where you expect. Binary search lets you ask 'is length X achievable' repeatedly, reducing a quadratic mess to something tractable. The substring and binary search topics intersect exactly here. During a live OA, if you can't immediately see the binary search wrapper, StealthCoder gives you the scaffold and lets you code forward.
Pattern tags
You know the problem.
Make sure you actually pass it.
Smallest Substring With Identical Characters II recycles across companies for a reason. It's hard-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 by a working Amazon engineer who got tired of watching qualified friends bomb OAs they'd solve cold in an IDE. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Smallest Substring With Identical Characters II interview FAQ
Is this as hard as the acceptance rate suggests?+
Yes. 41% pass rate means it's genuinely hard, not an easy problem badly explained. The trick isn't obvious and requires combining binary search with substring logic in a specific way. Most candidates get stuck on the greedy heuristic or brute force.
What's the trap that costs you the most time?+
Trying to build the answer iteratively instead of using binary search to narrow the length range first. Candidates often attempt O(n^2) or O(n^3) checks per position, which times out. The binary search frame is the unlock.
Does Salesforce still ask this one?+
It's in their reported problem set. Salesforce interviews emphasize string manipulation and algorithmic optimization, so substring and binary search questions are consistent with their style.
How do I know what character to reduce to in a given substring?+
The character with the maximum frequency in the substring window. Reducing all others down to match its count takes the fewest operations. Calculate the cost by summing the differences between that max frequency and each other character's count.
Can I skip binary search and just check every substring length sequentially?+
Technically yes, but it's slower. Binary search cuts the number of length candidates from n down to log(n), which matters on large inputs. Under time pressure in the real OA, linear scan may TLE while binary search won't.
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