剑指offer：重建二叉树

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* reConstructBinaryTree(vector<int> pre,vector<int> vin) {
        int vinlen = vin.size(); 
        if(vinlen == 0){
            return nullptr;
        }
        vector<int> pre_left, pre_right, vin_left, vin_right;
        TreeNode* head = new TreeNode(pre[0]);          //先序遍历的第一个是根节点.
        int ind = 0;                                  //找到中序遍历的根节点所在位置
        for(int i = 0; i < vinlen; i++){
            if(vin[i] == pre[0]){
                ind = i;
                break;
            }
        }
        //中序遍历，根节点左边是二叉树的左节点，右边是二叉树的右节点。
        //开始构建左子树vin[0] - vin[index]
        for(int i = 0; i < ind; i++){
            vin_left.push_back(vin[i]);
            pre_left.push_back(pre[i+1]);
        }
        //右子树
        for(int i = ind + 1; i < vinlen; i++){
            vin_right.push_back(vin[i]);
            pre_right.push_back(pre[i]);
        }
        head -> left = reConstructBinaryTree(pre_left, vin_left);
        head -> right = reConstructBinaryTree(pre_right, vin_right);
        return head;
                                }
};