1364C - Ehab and Prefix MEXs - CodeForces Solution

#include<bits/stdc++.h>
using namespace std;
int main() {
	ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
  	int n; cin >> n;
  	vector<int> a(n+1), b(n+1);
  	vector<bool> occ(n+1);
	for(int &p : b) p=-1;
	occ[0]=1;
	for(int i=1; i<=n; i++) {
		cin >> a[i], occ[a[i]]=1;
		if(a[i]!=a[i-1]) b[i]=a[i-1], occ[b[i]]=1;
	}
	int mex=0;
	for(int i=1; i<=n; i++) {
		while(occ[mex]) mex++;
		if(b[i]==-1) b[i]=mex, occ[mex]=1;
		cout << b[i] << ' ';
	} cout << '\n';
  	return 0;
}