给出树的前序遍历和后序遍历，构建树

必须知道一点：

前序遍历是：根、左、右

中序遍历是：左、根、右

后序遍历是：左、右、根

递归构建即可。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    struct TreeNode* reConstructBinaryTree(vector<int> pre,vector<int> in) {
        if(pre.size()==0)return NULL;
        TreeNode *root=new TreeNode(pre[0]);
        int p=0;
        while(in[p]!=(root->val))p++;
        root->left = reConstructBinaryTree(vector<int>(pre.begin()+1, pre.begin() + p+ 1),  vector<int>(in.begin(),in.begin() + p));
        root->right = reConstructBinaryTree(vector<int>(pre.begin() + p+ 1, pre.end() ) , vector<int>(in.begin() + p + 1, in.end() ));
        return root;
    }
};