Challenge 2: std::find With a Present and an Absent Value — Possible Solution
====================================================================

#include <iostream>
#include <vector>
#include <algorithm>

int main() {
    std::vector<int> nums = {5, 15, 25, 35};

    auto found = std::find(nums.begin(), nums.end(), 25);
    if (found != nums.end()) {
        std::cout << "Found" << std::endl;
    } else {
        std::cout << "Not found" << std::endl;
    }

    auto missing = std::find(nums.begin(), nums.end(), 99);
    if (missing != nums.end()) {
        std::cout << "Found" << std::endl;
    } else {
        std::cout << "Not found" << std::endl;
    }

    return 0;
}

Output:

    Found
    Not found

WHY THIS WORKS AS AN ANSWER
------------------------------
25 genuinely exists in nums, so std::find returns an iterator pointing
at it -- not equal to end() -- correctly reporting "Found." 99 doesn't
exist anywhere in nums, so std::find returns end() itself, which the
check `missing != nums.end()` correctly identifies as false, reporting
"Not found" -- exactly the end()-as-sentinel idiom the chapter
describes as used constantly throughout the STL.