二分找 24 要几步

在排好序的 [13, 15, 17, 23, 24] 里二分查找 24。运行下面这段程序，它数的是「比了几次」：

#include <iostream>
#include <string>
#include <vector>
using namespace std;

int bsearch_steps(const vector<int>& a, int target) {
    int lo = 0, hi = (int)a.size() - 1, steps = 0;
    while (lo <= hi) {
        int mid = (lo + hi) / 2;
        steps++;
        if (a[mid] == target) return steps;
        if (a[mid] < target) lo = mid + 1;
        else hi = mid - 1;
    }
    return steps;
}

int main() {
    cout << bsearch_steps({13, 15, 17, 23, 24}, 24) << endl;
}

（本题用 g++ -std=c++17 -O0 编译。）