LeetCode #889. Construct Binary Tree from Preorder and Postorder Traversal

题目描述：

Return any binary tree that matches the given preorder and postorder traversals.

Values in the traversals pre and post are distinct positive integers.

Example 1:

Input: pre = [1,2,4,5,3,6,7], post = [4,5,2,6,7,3,1]
Output: [1,2,3,4,5,6,7]

Note:

• 1 <= pre.length == post.length <= 30
• pre[] and post[] are both permutations of 1, 2, ..., pre.length.
• It is guaranteed an answer exists. If there exists multiple answers, you can return any of them.

class Solution {
public:
    TreeNode* constructFromPrePost(vector<int>& pre, vector<int>& post) {
        for(int i=0;i<post.size();i++) index[post[i]]=i;
        return construct_tree(pre,post,0,pre.size()-1,0,post.size()-1);
    }
    
    TreeNode* construct_tree(vector<int>& pre, vector<int>& post, int a, int b, int c, int d)
    {
        if(a>b) return NULL;
        if(a==b) return new TreeNode(pre[a]);
        int root_val=pre[a];
        int left_val=pre[a+1];
        int left_length=index[left_val]-c+1; // 左子树的节点数
        TreeNode* root=new TreeNode(root_val);
        root->left=construct_tree(pre,post,a+1,a+left_length,c,c+left_length-1);
        root->right=construct_tree(pre,post,a+left_length+1,b,c+left_length,d-1);
        return root;
    }
    
private:
    unordered_map<int,int> index; // 存储后序遍历值和下标之间的关系
};