Codeforces Round #353 (Div. 2) D. Tree Construction (二分，stl_set)

 1 #include <bits/stdc++.h>
 2 using namespace std;
 3 typedef pair <int , int> P;
 4 int a[int(1e5 + 5)] , ans[int(1e5 + 5)];
 5 set <P> v;
 6 int main()
 7 {
 8     int n;
 9     scanf("%d" , &n);
10     for(int i = 0 ; i < n ; ++i) {
11         if(i == 0) {
12             scanf("%d" , a + i);
13             v.insert(P(a[i] , i));
14             continue;
15         }
16         scanf("%d" , a + i);
17         auto it = v.upper_bound(P(a[i] , 0));
18         if(it == v.begin()) {
19             ans[i] = it->first;
20         }
21         else if(it == v.end()) {
22             ans[i] = (--it)->first;
23         }
24         else {
25             if(it->second > (--it)->second)
26                 ans[i] = (++it)->first;
27             else
28                 ans[i] = it->first;
29         }
30         v.insert(P(a[i] , i));
31     }
32     for(int i = 1 ; i < n - 1 ; ++i)
33         printf("%d " , ans[i]);
34     printf("%d\n" , ans[n - 1]);
35 }