private : unordered_map<int, int> mp;

TreeNode* buildTree_util(vector<int>& inorder, int i1, int i2, vector<int>& postorder, int p1, int p2){

if(i1 > i2 || p1 > p2)

return NULL;

TreeNode* root = new TreeNode(postorder[p2]);

int diff = mp[postorder[p2]] - i1;

root->left = buildTree_util(inorder, i1 , i1+diff-1, postorder, p1, p1+diff-1);

root->right = buildTree_util(inorder, i1+diff+1, i2, postorder, p1+diff, p2-1);

return root;

}

TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {

int n = inorder.size();

if(n == 0) return NULL;

for(int i = 0; i < n; i++)

mp[inorder[i]] = i; //(inorder, index)

return buildTree_util(inorder, 0, n-1, postorder, 0, n-1);

}