主要就是通过先序找到当前子树根节点，再用中序遍历分子树，不断递归下去。

#include <iostream>
#include <stdio.h>
#include <cstring>
#include <vector>
#include <cmath>
#include <algorithm>
#include <set>
#include <cassert>
#include <time.h>
#include <queue>
#include <map>
#include <stack>
#include <bitset>
#include <string>
#include <sstream>
#define INF 0x3f3f3f3f

using namespace std;

template <class Type>
Type stringToNum(const string& str)
{
    istringstream iss(str);
    Type num;
    iss >> num;
    return num;    
}

//======================================================

typedef struct node
{
    int m_key;
    node * m_pLeft,*m_pRight;
}BinaryTreeNode;

//BinaryTreeNode* constructTree(int* preOrder,int preLen,int* sMidOrder, int midLen){
BinaryTreeNode* constructRoot(int* preOrder, int* midOrder, int len){

    //先根遍历（前序遍历）的第一个值就是根节点的key
    int rootKey=preOrder[0];
    BinaryTreeNode* root=new BinaryTreeNode;
    root->m_key=rootKey;
    root->m_pLeft=root->m_pRight=NULL;
    if(len==1 && *preOrder==*midOrder)//只有一个节点
        return root;

    //在中根遍历（中序遍历）中找到根节点
    int* rootMidOrder=midOrder;
    int leftLen=0; //左子树节点数
    while(*rootMidOrder!=rootKey&&rootMidOrder<=(midOrder+len-1)){ 
        ++rootMidOrder;
        ++leftLen;
    }
    if(*rootMidOrder!=rootKey)//在中根序列未找到根节点，输入错误
        return NULL;

    if(leftLen>0){ //构建左子树
        root->m_pLeft=constructRoot(preOrder+1,midOrder,leftLen);
    }
    if(len-leftLen-1>0){ //构建右子树
        root->m_pRight=constructRoot(preOrder+leftLen+1,rootMidOrder+1,len-leftLen-1);
    }
    return root;
}

BinaryTreeNode* construct(int* preOrder,int* midOrder,int len){
    if(preOrder==NULL||midOrder==NULL||len<=0)
        return NULL;
    return constructRoot(preOrder,midOrder,len);
}

//先根遍历
void preOrderRecursionPrint(BinaryTreeNode* root){
    if(root==NULL)
        return;
    cout<<root->m_key<<endl;   //visit
    preOrderRecursionPrint(root->m_pLeft);
    preOrderRecursionPrint(root->m_pRight);
}

int main()
{
    //freopen("input.txt","r",stdin);

    //先根序列
    int preOrder[8]={1,2,4,7,3,5,6,8};
    //中根序列
    int midOrder[8]={4,7,2,1,5,3,8,6};

    BinaryTreeNode * treeRoot = construct(preOrder,midOrder,8);

    preOrderRecursionPrint(treeRoot);

    return 0;
}

输出如下：