根据先序序列、中序序列建立二叉树,给出cpp代码示例

以下是根据先序序列和中序序列构建二叉树的C++代码示例：

#include<iostream>
#include<unordered_map>
using namespace std;

struct TreeNode {
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(NULL), right(NULL) {}
};

class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        unordered_map<int, int> map;
        for(int i=0; i<inorder.size(); i++) {
            map[inorder[i]] = i;
        }
        return buildTreeHelper(preorder, 0, preorder.size()-1, inorder, 0, inorder.size()-1, map);
    }

    TreeNode* buildTreeHelper(vector<int>& preorder, int preStart, int preEnd, vector<int>& inorder, int inStart, int inEnd, unordered_map<int, int>& map) {
        if(preStart > preEnd || inStart > inEnd) {
            return NULL;
        }
        int rootVal = preorder[preStart];
        TreeNode* root = new TreeNode(rootVal);
        int rootIndex = map[rootVal];
        int leftSubtreeSize = rootIndex - inStart;
        root->left = buildTreeHelper(preorder, preStart+1, preStart+leftSubtreeSize, inorder, inStart, rootIndex-1, map);
        root->right = buildTreeHelper(preorder, preStart+leftSubtreeSize+1, preEnd, inorder, rootIndex+1, inEnd, map);
        return root;
    }
};

其中，`buildTree`函数接收先序序列和中序序列，返回构建完成的二叉树。`buildTreeHelper`函数是递归函数，用于构建二叉树的子树。通过使用哈希表记录中序序列中每个元素的下标，可以快速查找根节点在中序序列中的位置，从而方便地划分左右子树。