STL - General Questions

Which container gives O(1) average lookup by key?

What do STL iterators provide?

What will be the output of the program?

#include <iostream>
using namespace std;
#include <vector>
#include <algorithm>
int main() {
    vector<int> v = {5, 2, 9, 1};
    sort(v.begin(), v.end());
    for (int x : v) cout << x << " ";
    cout << endl;
    return 0;
}

What will be the output of the program?

#include <iostream>
#include <string>
using namespace std;
#include <map>
int main() {
    map<string, int> m;
    m["b"] = 2; m["a"] = 1; m["b"] += 5;
    for (auto &p : m) cout << p.first << p.second << " ";
    cout << endl;
    return 0;
}

What will be the output of the program?

#include <iostream>
using namespace std;
#include <vector>
#include <algorithm>
int main() {
    vector<int> v = {3, 1, 2};
    reverse(v.begin(), v.end());
    for (int x : v) cout << x;
    cout << endl;
    return 0;
}