Binary Tree From Inorder And Postorder

• 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(unordered_map<int,int>& mp, vector<int>& B, int i, int l, int r) {
    if(l > r) return NULL;
    TreeNode* res = new TreeNode(B[i]);
    if(l != r) {
        int mid = mp[B[i]];
        res->left = helper(mp, B, i - (r - mid) - 1, l, mid - 1);
        res->right = helper(mp,B, i - 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 < A.size(); i++) mp[A[i]] = i;
    return helper(mp, B, B.size() - 1, 0, A.size() - 1);
}