105. 从前序与中序遍历序列构造二叉树

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给定两个整数数组 preorder 和 inorder ，其中 preorder 是二叉树的先序遍历， inorder 是同一棵树的中序遍历，请构造二叉树并返回其根节点。

示例 1:

输入: preorder = [3,9,20,15,7], inorder = [9,3,15,20,7]
输出: [3,9,20,null,null,15,7]

示例 2:

输入: preorder = [-1], inorder = [-1]
输出: [-1]

提示:

• 1 <= preorder.length <= 3000
• inorder.length == preorder.length
• -3000 <= preorder[i], inorder[i] <= 3000
• preorder 和 inorder 均 无重复 元素
• inorder 均出现在 preorder
• preorder 保证 为二叉树的前序遍历序列
• inorder 保证 为二叉树的中序遍历序列

 

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
    unordered_map<int,int>inorderId;
public:
    TreeNode* buildTree(vector<int>& preorder,vector<int>& inorder,int preorder_left,int preorder_right,int inorder_left,int inorder_right){
        if(preorder_left>preorder_right) return nullptr;
        int rootValue=preorder[preorder_left];
        int root_inorder=inorderId[rootValue];
        int leftTreeSize=root_inorder-inorder_left;
        TreeNode* newRoot=new TreeNode(rootValue);
        newRoot->left=buildTree(preorder,inorder,preorder_left+1,preorder_left+leftTreeSize,inorder_left,root_inorder-1);
        newRoot->right=buildTree(preorder,inorder,preorder_left+leftTreeSize+1,preorder_right,root_inorder+1,inorder_right);
        return newRoot;
    }

    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        int n=preorder.size();
        for(int i=0;i<n;i++) inorderId[inorder[i]]=i;
        return buildTree(preorder,inorder,0,n-1,0,n-1);
    }
};