Construct Binary Tree From Inorder And Preorder

• Time : $O(n)$
• Space : $O(n)$

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
TreeNode* helper(vector<int>& A, unordered_map<int, int>& B, int i, int l, int r) {
    if(l > r) return NULL;
    TreeNode* res = new TreeNode(A[i]);
    if(l != r) {
        int mid = B[A[i]];
        res->left = helper(A,B,i + 1, l, mid - 1);
        res->right = helper(A,B,i + mid - l + 1, mid + 1, r);
    }
    return res;
}
TreeNode* Solution::buildTree(vector<int> &A, vector<int> &B) {
    unordered_map<int, int> mp;
    for(int i = 0; i < B.size(); i++) mp[B[i]] = i;
    return helper(A,mp,0,0,B.size() - 1);
}