二叉树由先序和中序建树

递归

#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <vector>
#include <algorithm>
#include <string>
using namespace std;

template <class T>
struct Node {
    T data;
    Node *left;
    Node *right;

    Node() : data(), left(NULL), right(NULL) {}    // data()将使内置类型对象也初始化为0
};

/*
** 根据二叉树的先序遍历序列与中序遍历序列建立二叉树。
** 
** head:        建立的树的根的地址
** pre_begin:    先序遍历序列的首位置
** pre_end:        先序遍历序列的尾位置的下一位置
** in_begin:    中序遍历序列的首位置
*/
template <class T, class Iter>
void creat_tree(Node<T> *&head,
    const Iter pre_begin, const Iter pre_end, const Iter in_begin) {
    if (pre_begin == pre_end) {
        head = NULL;
        return ;
    }

    const Iter in_end = in_begin + (pre_end - pre_begin);

    head = new Node<T>();
    // 先设置根
    head->data = *pre_begin;    
    
    // 再建左子树
    const Iter in_middle = find(in_begin, in_end, *pre_begin);
    const Iter left_in_begin = in_begin;
    const Iter left_in_end = in_middle;
    const Iter left_pre_begin = pre_begin + 1;
    const Iter left_pre_end = left_pre_begin + (left_in_end - left_in_begin);
    creat_tree(head->left, left_pre_begin, left_pre_end, left_in_begin);

    // 后建右子树
    const Iter right_in_begin = in_middle + 1;
    const Iter right_in_end = in_end;
    const Iter right_pre_begin = left_pre_end;
    const Iter right_pre_end = pre_end;
    creat_tree(head->right, right_pre_begin, right_pre_end, right_in_begin);
}

template <class T>
void free_tree(Node<T> *&root) {
    if (NULL == root) {
        return ;
    }

    free_tree(root->left);
    free_tree(root->right);
    free(root);
    root = NULL;
}

template <class T>
void post_order(Node<T> *root) {
    if (NULL == root) {
        return ;
    }

    post_order(root->left);
    post_order(root->right);
    cout << root->data << endl;
}

template <class T>
void input (vector<T> &v, int size) {
    T value;
    while(size--) {
        cin >> value;
        v.push_back(value);
    }
}

int main(int argc, char **argv) {
    freopen("cin.txt", "r", stdin);

    typedef char T;
    vector<T> pre;
    vector<T> in;
    int size;
    cin >> size;
    input(pre, size);
    input(in, size);

    Node<T> *head = NULL;
    creat_tree(head, pre.begin(), pre.end(), in.begin());

    cout << "Post-order:" << endl;
    post_order(head);

    free_tree(head);
    return 0;
}

输入样例：

4
A B C D
B A D C