LeetCode Construct Binary Tree from Inorder and Postorder Traversal

class Solution {
public:
TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
int ilen = inorder.size();
int plen = postorder.size();

if (ilen == 0 || plen == 0) return NULL;
return dfs(inorder, postorder, 0, ilen, plen - 1, -1);
}

TreeNode* dfs(vector<int> &inorder, vector<int> &postorder, int is, int ie, int ps, int pe) {

if (is >= ie || ps <= pe) return NULL;
int rval = postorder[ps];
int iroot= is;

while (iroot != ie && inorder[iroot] != rval) iroot++;

int left_len = iroot - is;

TreeNode* nroot = new TreeNode(rval);

nroot->left = dfs(inorder, postorder, is, iroot, pe + left_len, pe);
nroot->right= dfs(inorder, postorder, iroot + 1, ie, ps - 1, pe + left_len);

return nroot;
}
};