UVA 536 Tree Recovery （二叉树重建）

题意：已知先序中序，输出后序。

#pragma comment(linker, "/STACK:102400000, 102400000")
#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cctype>
#include<cmath>
#include<iostream>
#include<sstream>
#include<iterator>
#include<algorithm>
#include<string>
#include<vector>
#include<set>
#include<map>
#include<stack>
#include<deque>
#include<queue>
#include<list>
#define Min(a, b) ((a < b) ? a : b)
#define Max(a, b) ((a < b) ? b : a)
typedef long long ll;
typedef unsigned long long llu;
const int INT_INF = 0x3f3f3f3f;
const int INT_M_INF = 0x7f7f7f7f;
const ll LL_INF = 0x3f3f3f3f3f3f3f3f;
const ll LL_M_INF = 0x7f7f7f7f7f7f7f7f;
const int dr[] = {0, 0, -1, 1, -1, -1, 1, 1};
const int dc[] = {-1, 1, 0, 0, -1, 1, -1, 1};
const int MOD = 1e9 + 7;
const double pi = acos(-1.0);
const double eps = 1e-8;
const int MAXN = 26 + 10;
const int MAXT = 10000 + 10;
using namespace std;
char pre_order[MAXN];
char in_order[MAXN];
int leftchild[MAXN];
int rightchild[MAXN];
int build(int L1, int R1, int L2, int R2){
    if(L1 > R1) return 0;
    int root = pre_order[L1] - 'A' + 1;
    int st = L2;
    while((in_order[st] - 'A' + 1) != root) ++st;
    int cnt = st - L2;
    leftchild[root] = build(L1 + 1, L1 + cnt, L2, L2 + cnt - 1);
    rightchild[root] = build(L1 + 1 + cnt, R1, st + 1, R2);
    return root;
}
void dfs(int root){
    if(leftchild[root]) dfs(leftchild[root]);
    if(rightchild[root]) dfs(rightchild[root]);
    printf("%c", root + 'A' - 1);
}
int main(){
    while(scanf("%s", pre_order) != EOF){
        scanf("%s", in_order);
        int len = strlen(pre_order);
        build(0, len - 1, 0, len - 1);
        int root = pre_order[0] - 'A' + 1;
        dfs(root);
        printf("\n");
    }
    return 0;
}