106. 从中序与后序遍历序列构造二叉树

106. 从中序与后序遍历序列构造二叉树

参考105. Construct Binary Tree from Preorder and Inorder Traversal | Cinte's Leetcode Record

/**
 * 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 {
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        unordered_map<int, int> map;
        int sz = inorder.size();
        for(int i = 0; i < sz; ++i)
            map[inorder[i]] = i;
        int index = sz - 1;
        return build(map, postorder, 0, sz - 1, index);
    }
private:
    TreeNode* build(unordered_map<int, int>& inorder, vector<int>& postorder, int lo, int hi, int& index)
    {
        if(lo > hi)
            return nullptr;
        int val = postorder[index--];
        TreeNode* root = new TreeNode(val);
        root->right = build(inorder, postorder, inorder[val] + 1, hi, index);
        root->left = build(inorder, postorder, lo, inorder[val] - 1, index);
        return root;
    }
};