4.重建二叉树——剑指offer

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) {}
 * };
 */
//递归思想
//前序遍历：中左右     中序遍历：左中右
//通过前序的第一个值找到中心结点，得到前序和中序的左子树和又子树
//递归左子树和又子树
class Solution {
public:
    TreeNode* reConstructBinaryTree(vector<int> pre,vector<int> vin) {
    int length = pre.size();
        if(length == 0) return NULL;
    int root = pre[0];
        int index;
        vector<int> pre_left , pre_right , vin_left , vin_right;
        for(int i = 0;i < length; ++i){
        if(root == vin[i]){
            index = i;
            break;
        }
        }
        for(int i = 0; i < index; ++i){
            pre_left.push_back(pre[i+1]);
            vin_left.push_back(vin[i]);
        }
        for(int i = index + 1;i < length; ++i){
            pre_right.push_back(pre[i]);
            vin_right.push_back(vin[i]);
        }
        TreeNode* node = new TreeNode(root);
        node->left = reConstructBinaryTree(pre_left , vin_left);
        node->right = reConstructBinaryTree(pre_right , vin_right);
        return node;
    }
};