【暑假】根据in_order与post_order构树

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<sstream>
 4 #include<algorithm>
 5 #define FOR(a,b,c) for(int a=(b);a<(c);a++)
 6 using namespace std;
 7 
 8 const int maxn = 10000 + 10,INF=1<<30;
 9 int in_order[maxn],post_order[maxn],lch[maxn],rch[maxn];
10 int n,best,ans,root;
11 
12 int read_list(int* a){
13     string line;
14     if(!getline(cin,line)) return false;
15     stringstream ss(line);
16     n=0;
17     int x;
18     while(ss>>x) a[n++]=x;
19     return n>0;
20 }
21 
22 int build_tree(int L1,int R1,int L2,int R2){
23     if(R1<L1) return 0;  
24     int root=post_order[R2];
25     int p=L1;
26     while(in_order[p] != root) p++; 
27     int cnt=p-L1;
28     lch[root]=build_tree(L1,p-1,L2,L2+cnt-1);
29     rch[root]=build_tree(p+1,R1,L2+cnt,R2-1);
30     return root;     
31 }
32 
33 void dfs(int u,int cnt){ 
34     cnt += u; 
35     if(cnt>ans) return;
36     if(!lch[u] && !rch[u]) 
37       if(cnt<ans || (cnt==ans && best<u)){ ans=cnt; best=u;}
38     if(lch[u]) dfs(lch[u],cnt);
39     if(rch[u]) dfs(rch[u],cnt);
40 }
41 
42 int main(){
43     while(read_list(in_order)){
44         read_list(post_order);
45         build_tree(0,n-1,0,n-1);
46         ans=INF;
47         dfs(post_order[n-1],0); 
48         cout<<best<<endl;
49     }
50     return 0;
51 }