ambiguous runtime error causing the program to crash immediately - c++

Question

#include <bits/stdc++.h>
using namespace std;
int freq[101034];
int main() {
int n;
cin >> n;
set<int> st;
for (int i = 0; i < n; i++) {
int x;
cin >> x;
freq[x]++;
st.insert(x);
}
while (!st.empty()) {
for (auto x : st) {
if (freq[x] <= 0) {
st.erase(x);
continue;
}
cout << x << ' ';
freq[x]--;
}
cout << '\n';
}
return 0;
}
The problem I have tried to solve: Given an array of integers n up to 10^5 and each element up to 10^5, the task is to print the array sorted without repetition, then delete the array elements that are printed, then repeat until the array is empty.
For instance if array [1, 1, 2, 3, 4, 4]
This should be printed
1 2 3 4
1 4
I maintained a frequency array to hold each element frequency and the code above causes runtime error. The program crashes. I've tried to remove the if statement, the program runs normally, but goes to infinite loop for sure! I really can't figure why the if causes runtime error.

Answer 1

The issue is in this snippet:
while (!st.empty()) {
for (auto x : st) {
if (freq[x] <= 0) {
st.erase(x);
continue;
}
cout << x << ' ';
freq[x]--;
}
cout << '\n';
}
The range-based for loop uses iterators in the back (see this for more details). When you erase x from st the loop iterator (pointing to x) becomes invalid (this means that you mustn't use it anymore), but in the snippet above, it still gets incremented at the end of the loop in the background, that result undefined behavior, hence the runtime error.
Take a look at this page to see how you should implement it properly. Applying the practice of the previous link to your code:
while (!st.empty()) {
for (auto it = cbegin(st); it != cend(st);) {
auto x = *it;
if (freq[x] <= 0) {
it = st.erase(it);
} else {
++it;
cout << x << ' ';
freq[x]--;
}
}
cout << '\n';
}