算法-二叉树-从中序与后序遍历序列构造二叉树（递归）

/**
 * 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 {
    //遍历序列
    int post_idx;
    //哈希表
    unordered_map<int, int>umap;
public:
    TreeNode* helper(int in_left, int in_right, vector<int>& inorder, vector<int>& postorder){
        //若空
        if(in_left > in_right) return nullptr;
        int root_val = postorder[post_idx];
        //创建节点
        TreeNode* root = new TreeNode(root_val);
        int index = umap[root_val];
        post_idx--;
        //右子树
        root->right = helper(index + 1, in_right, inorder, postorder);
        //左子树
        root->left = helper(in_left, index - 1, inorder, postorder);
        return root;
    }
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        //开始序号
        post_idx = postorder.size() - 1;
        //
        int idx = 0;
        for(auto& innode : inorder){
            umap[innode] = idx++;
        }
        //递归
        return helper(0, postorder.size() - 1, inorder, postorder);
    }
};