样例都过不了，不输出

#include<bits/stdc++.h>
using namespace std;
int main(){
    int n,m;
    cin >> n >> m;
    int b[1005] = {0};
    for(int i = 0;i<m;i++){
        int a;
        cin >> a;
        b[a] = 1;
    }
    int f = 0;
    for(int i = 0;i<n;i++){
        if(b[i] == 0) cout << i << " "; f = 1;
    }
    if(f == 0) cout << n;
    return 0;
}

不知道为啥