leetcode 105 Construct Binary Tree from Preorder and Inorder Traversal

题目

前序+中序->建树

解题思路

思路 01

前序确定根（第一个），中序用根分割左根右

思路 02

借助hashmap，优化每次中序中需要查找根索引的操作

Code

Code 01

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */

class Solution {
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        return generate_tree(preorder, inorder, 0, preorder.size()-1, 0, inorder.size()-1);
    }
private:
    TreeNode * generate_tree(vector<int>& preorder, vector<int>& inorder, int pl, int pr, int il, int ir){
        if(il>ir)return NULL;
        int k=il;
        while(k<ir&&inorder[k]!=preorder[pl])k++; //中序中找到当前根节点
        TreeNode* root=new TreeNode(preorder[pl]);
        root->left=generate_tree(preorder,inorder,pl+1,pl+(k-il),il, k-1);
        root->right=generate_tree(preorder,inorder,pl+(k-il)+1,pr,k+1, ir);
        return root;
    }
};