Sets and maps usage

std::set and std::map keep keys sorted in a balanced tree—O(log n) operations. Use when you need ordered traversal or guaranteed log time without hash assumptions.

Container guide

• unordered_set — unique keys, average O(1)
• set — unique sorted keys, O(log n)
• map — sorted key → value
• unordered_map — hash key → value

Duplicate detection

#include 
#include 
#include 

int main() {
    std::vector a = {1, 2, 3, 2, 4};
    std::unordered_set seen;
    for (int x : a) {
        if (seen.count(x)) {
            std::cout << "duplicate: " << x << "\n";
            break;
        }
        seen.insert(x);
    }
    return 0;
}

Ordered map iteration

std::map iterates keys in order—useful for sweep-line and coordinate compression previews.

Important interview questions and answers

1. Q: set vs unordered_set?
   A: set sorted O(log n); unordered_set average O(1) unsorted.
2. Q: multiset?
   A: Allows duplicate keys with count—frequency beyond simple set.

Self-check

1. When pick map over unordered_map?
2. How detect first duplicate in O(n) average?

Pitfall: Using map when you only need fast lookup—unordered_map may be faster average case.