从中序和前序遍历序列构造二叉树

题目链接

class Solution {
public:unordered_map<int,int> hash;TreeNode* build(int rooti,int left,int right,vector<int>& preorder){if(left > right)return nullptr;TreeNode* root = new TreeNode(preorder[rooti]);int index = hash[preorder[rooti]];root->left = build(rooti+1,left,index-1,preorder);root->right = build(rooti+1+index-left,index+1,right,preorder);return root;}TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {for(int i = 0;i < inorder.size();i++){hash[inorder[i]] = i;}return build(0,0,preorder.size()-1,preorder);}
};