1144 The Missing Number

 1 #include<iostream>
 2 #include<map>
 3 using namespace std;
 4 
 5 int main() {
 6     int n,t,ans = 1;
 7     map<int,bool> mp;
 8     cin>>n;
 9     for(int i = 0; i < n; ++i) {
10         scanf("%d",&t);
11         if(t >= 1&&mp[t] == false) {
12             mp[t] = true;
13             if(t == ans) {
14                 while(mp[ans]) {
15                     ans++;
16                 }
17             }
18         }
19     }
20     printf("%d",ans);
21     return 0;
22 }