Implement binary search on a sorted array
Reported in Cisco USA engineering loops. Foundational search algorithm with clear loop invariants and edge cases.
Interview scenario
Context for Cisco candidates:
Return the index of target or -1 if absent. Discuss iterative vs recursive variants.
Model answer
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How to frame this at Cisco: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Binary search halves the search space each step by comparing target with the middle element. Maintain lo and hi bounds; loop while lo <= hi.
function binarySearch(arr, target) {
let lo = 0, hi = arr.length - 1;
while (lo <= hi) {
const mid = lo + Math.floor((hi - lo) / 2);
if (arr[mid] === target) return mid;
if (arr[mid] < target) lo = mid + 1;
else hi = mid - 1;
}
return -1;
}Complexity: O(log n) time, O(1) space iterative. Common bugs: integer overflow on (lo+hi)/2 in languages without safe mid calculation, infinite loops from incorrect bounds, off-by-one on empty arrays.
Extensions: first/last occurrence (lower/upper bound), search rotated array, and search in answer space (binary search on value, not index).
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