105

/**
 * 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 {
unordered_map<int, int> inorderIndex;
int curr = 0;
public:
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        for(int i=0; i<inorder.size(); i++)
            inorderIndex[inorder[i]] = i;

        return constructTree(preorder, 0, inorder.size()-1);
    }

    TreeNode* constructTree(vector<int> &preorder, int left, int right){
        if(left > right) return NULL;

        int val = preorder[curr++];
        TreeNode *root = new TreeNode(val);
        root->left = constructTree(preorder, left, inorderIndex[val]-1);
        root->right = constructTree(preorder, inorderIndex[val]+1, right);
        return root;
    }

};