Binary Search (Recursive)

Use the same binary-search window as the iterative lesson, but pass lo and hi through recursive calls.

Algorithm

Basic Implementation

basic.cpp

#include <iostream>
#include <unordered_map>
#include <vector>

int search(const std::vector<int>& arr, int target, int lo, int hi) {
    if (lo > hi) return -1;
    int mid = lo + (hi - lo) / 2;
    if (arr[mid] == target) return mid;
    if (arr[mid] < target) return search(arr, target, mid + 1, hi);
    return search(arr, target, lo, mid - 1);
}

int main() {
    std::vector<int> arr{1, 3, 5, 7, 9, 11, 13};
    int target = 11;
    std::cout << search(arr, target, 0, static_cast<int>(arr.size()) - 1) << "\n";
    return 0;
}

Complexity

Time: O(log n)
Space: O(log n) call stack

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `search-binary-recursive`.

cross-language comparison This C++ DSA version keeps the same data and final output as every other DSA book in this wave.