0106. Construct Binary Tree from Inorder and Postorder Traversal.md

/**
 * 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;
        for(int i = 0; i < inorder.size(); i++) {
            map[inorder[i]] = i;
        }
        int postidx = postorder.size() - 1;

        function<TreeNode*(int,int)> dfs = [&](int l, int r) -> TreeNode* {
            if(l > r) return nullptr;
            int val = postorder[postidx];
            postidx--;
            TreeNode* root = new TreeNode(val);
            int mid = map[val];

            root->right = dfs(mid + 1, r);
            root->left = dfs(l, mid - 1);
            return root;
        };

        return dfs(0, postorder.size() - 1);
    }
};