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;
    }
};