剑指Offer——重建二叉树——C++

重建二叉树

题目描述

输入某二叉树的前序遍历和中序遍历的结果，请重建出该二叉树。假设输入的前序遍历和中序遍历的结果中都不含重复的数字。例如输入前序遍历序列{1,2,4,7,3,5,6,8}和中序遍历序列{4,7,2,1,5,3,8,6}，则重建二叉树并返回。

思路：根据前序第一个字符是根的特性，再在中序中找到这个位置，分开，左边的是左子树，右边的是右子树。然后递归求出结果。

/**

 * 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) {

        int len = in.size();

        if(len == 0){

            return NULL;

        }

        vector<int> left_pre, right_pre;

        vector<int> left_in, right_in;

        TreeNode *head = new TreeNode(pre[0]);

        int index = 0;

        for(int i=0; i<len; i++){

            if(in[i] == pre[0]){

                index = i;

                break;

            }

        }

         

        for(int i=0; i<index; i++){

            left_pre.push_back(pre[i+1]);

            left_in.push_back(in[i]);

        }

        for(int i=index+1; i<len; i++){

            right_pre.push_back(pre[i]);

            right_in.push_back(in[i]);

        }

         

        //TreeNode* root = new TreeNode(pre[0]);

        head->left = reConstructBinaryTree(left_pre, left_in);

        head->right = reConstructBinaryTree(right_pre, right_in);       

         

        return head;

    }

};