leetcode 106 Construct Binary Tree from Inorder and Postorder Traversal

题目

中序+后序->建树

思路

思路01

后序确定根（最后一个），中序中用根分割左中右

思路02

利用hashmap，优化中序中每次查找根索引的操作

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