剑指offer: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。

/**
 * 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()!=in.size()){
            return NULL;
        }

        if(pre.size()==0){
            return NULL;
        }

        return helper(pre,0,pre.size()-1,in,0,in.size()-1);
    }
private:

    TreeNode* helper(vector<int> &pre,int preL,int preR,vector<int> &in,int inL,int inR){

     //   if(preL<=preR && inL<=inR){
            TreeNode* root=new TreeNode(pre[preL]);

            int mid=inL;
            while(in[mid]!=pre[preL]) mid++;
            int cnt=mid-inL;

            if(inL==mid){
                root->left=NULL;
            }else{
                root->left=helper(pre,preL+1,preL+cnt,in,inL,mid-1);
            }

            if(inR==mid){
                root->right=NULL;
            }else{
                root->right=helper(pre,preL+cnt+1,preR,in,mid+1,inR);
            }

            return root;
      //  }

    }

};

注意左右子树为null时的判断！！