首先要知道一个结论，前序/后序+中序序列可以唯一确定一棵二叉树，所以自然而然可以用来建树。

看一下前序和中序有什么特点，前序1,2,4,7,3,5,6,8 ，中序4,7,2,1,5,3,8,6；

有如下特征：

代码如下

class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        return pre_order(0, inorder.size() - 1, 0, inorder.size() - 1, preorder, inorder);
    }
    
    TreeNode *pre_order(int leftpre, int rightpre, int leftin, int rightin, vector<int> &pre, vector<int> &in) {
        if (leftpre > rightpre || leftin > rightin) return NULL;
        TreeNode *root = new TreeNode(pre[leftpre]);
        int rootin = leftin;
        while (rootin <= rightin && in[rootin] != pre[leftpre]) rootin++;
        int left = rootin - leftin;
        root->left = pre_order(leftpre + 1, leftpre + left, leftin, rootin - 1, pre, in);
        root->right = pre_order(leftpre + left + 1, rightpre, rootin + 1, rightin, pre, in);
        return root;
    }
};