Interview Question

Front End Engineer Interview

-Folsom, CA


If you have 500 revisions of a program, write a program that will find and return the FIRST bad revision given a isBad(revision i) function.


Interview Answers

7 Answers


function findBadVersion(n) { let min = 0,max = n, mid; while(min < max){ mid = min + Math.floor((max-min)/2); if(isBadVersion(mid)){ max = mid; }else{ min = mid+1; } } return min; };

Anonymous on


I don't understand how a Binary Search would work in this case; given just an array of revisions, and a pure function that simply takes one revision, and returns true or false. Based on the Binary Search being akin to `git bisect`, the docs state, "You use it by first telling it a "bad" commit that is known to contain the bug, and a "good" commit that is known to be before the bug was introduced." - I don't see that we already know which revision is bad (or one that is good), as that is what is being asked of us to find. It would seem to me a breakable loop would solve this. ``` console.clear(); const testFind = false, // [true|false] For testing purposes only. isBad = rev => testFind, // Stub for given requirement (would otherwise import). findBadFirst = (revisions) => { // Using () for clarity and consistency. const revFound = { 'found': false }; // Using object to avoid a Type conflict. for (let r = 0; r < revisions.length; r++) { // Using `for` so we can break. if (isBad(revisions[r])) { revFound.found = true; revFound['revision'] = revisions[r]; break; } } return revFound; }; // Testing... const store = { 'revisions': [ { 'item': 1 } ] }, foundOut = findBadFirst(store.revisions); console.log('Found: ', foundOut.found); console.log('Rev: ', foundOut.revision ? foundOut.revision : 'None (all good!)'); ```

Keith D Commiskey on


```` const findBadRevision = (start, end)=>{ if(start>end) return -1; let mid = parseInt((start+end)/2); if(isBad(mid)){ if(mid==0 || (!isBad(mid-1))) return mid; return findBadRevision(start, mid); } return findBadRevision(mid+1, end); } ````

Kal on


Vanilla binary search like some. folks have posted is wrong. It doesnt. return the min version when a bad commit occurred. If b==bad, g==good take this sequence [gbbb]. midpoint=2. Any nswer which checks isBad(mid) && !isBad(mid-1) before returning mid will work

smk on


Yeah, binary search. very similar to what git bisect does.

Anonymous on


Hashmap? Since we are already provided with a key, it takes constant time to lookup using the key.

Gantavya on


Binary Search algorithm.

Anonymous on

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