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Construct Binary Tree from Inorder and Postorder Traversal⚓︎

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Description⚓︎

Given two integer arrays inorder and postorder where inorder is the inorder traversal of a binary tree and postorder is the postorder traversal of the same tree, construct and return the binary tree.

Example 1:

  • Input: inorder = [9,3,15,20,7], postorder = [9,15,7,20,3]
  • Output: [3,9,20,null,null,15,7]

Example 2:

  • Input: inorder = [-1], postorder = [-1]
  • Output: [-1]

Constraints:

  • 1 <= inorder.length <= 3000
  • postorder.length == inorder.length
  • -3000 <= inorder[i], postorder[i] <= 3000
  • inorder and postorder consist of unique values.
  • Each value of postorder also appears in inorder.
  • inorder is guaranteed to be the inorder traversal of the tree.
  • postorder is guaranteed to be the postorder traversal of the tree.

Solution⚓︎

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/**
 * 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 {
private:
    unordered_map<int, int> pos;

    TreeNode* treeBuilder(vector<int>& inorder, vector<int>& postorder, int inL, int inR, int postL, int postR) {
        if (inL > inR) return nullptr;
        auto root = new TreeNode(postorder[postR]);
        int index = pos[root->val];

        root->left = treeBuilder(inorder, postorder, 
                                inL, index - 1,
                                postL, postL + index - 1 - inL);
        root->right = treeBuilder(inorder, postorder,
                                index + 1, inR,
                                postL + index - inL, postR - 1);
        return root;
    }

public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        int treeSize = inorder.size();
        for (int i = 0; i < treeSize; i++) pos[inorder[i]] = i;
        return treeBuilder(inorder, postorder, 0, treeSize - 1, 0, treeSize - 1);
    }
};