LeetCode Construct Binary Tree from Inorder and Postorder Traversal

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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;    }};