/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* build(const vector<int>& inorder, int inStart, int inEnd, const vector<int>& preorder, int preStart, int preEnd, unordered_map<int,int>& inMap){
        if(inStart > inEnd || preStart > preEnd){
            return nullptr;
        }

        int rootVal = preorder[preStart];
        TreeNode* root = new TreeNode(rootVal);
        int inRoot = inMap[rootVal];
        int numsLeft = inRoot - inStart;

        root->left = build(inorder, inStart, inRoot - 1, preorder, preStart + 1, preStart + numsLeft, inMap);
        root->right = build(inorder, inRoot + 1, inEnd, preorder, preStart + numsLeft + 1, preEnd, inMap);

        return root;
    }
    TreeNode* buildTree(vector<int>& preorder, vector<int>& inorder) {
        unordered_map<int,int> inMap;
        for(int i = 0; i < inorder.size(); ++i){
            inMap[inorder[i]] = i;
        }
        TreeNode* root = build(inorder, 0, inorder.size() - 1, preorder, 0, preorder.size() - 1, inMap);
        return root;

    }
};