[剑指Offer] 4.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】递归，先找到根结点，再找到左右子树的前序与中序序列进行递归

/**
 * 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> in) {
        
        if(pre.size() == 0)
            return NULL;
        TreeNode* Root = new TreeNode(pre[0]);
        
        vector<int> pre1,pre2;
        vector<int> in1,in2;
        int root = pre[0];
        int root_id = 0;
        
        //找到根结点在中序序列中位置root_id
        for(int i = 0;i < in.size();i ++){
            if(root == in[i]){
                root_id = i;
            }
        }
        
        for(int i = 0;i < root_id;i ++)
            in1.push_back(in[i]);
        for(int i = root_id + 1;i < in.size();i ++)
            in2.push_back(in[i]);
        
        for(int i = 1;i < in1.size() + 1;i ++)
            pre1.push_back(pre[i]);
        for(int i = in1.size() + 1;i < pre.size();i ++)
            pre2.push_back(pre[i]);
        
        Root->left = reConstructBinaryTree(pre1,in1);
        Root->right = reConstructBinaryTree(pre2,in2);
        
        return Root;
        
    }
};