【leetcode】Construct Binary Tree from Inorder and Postorder Traversal

TreeNode *addNode(vector<int> &inorder, int s1, int end1, vector<int> &postorder,  int s2, int end2)
    {
        if(s1 > end1 || s2 > end2)
        return NULL;
        //construct the root 
        TreeNode *root = new TreeNode(postorder[end2]);
        //index_of_root: the index of root in inorder.
        int i = s1;
        for(; i <= end1; ++i)
        {
            if(inorder[i] == postorder[end2])
               break;
        }
        if(i > end1)
            return NULL;
        int dist = i - s1;
        //construct the left branch
        root->left = addNode(inorder, s1, i - 1, postorder, s2, s2 + dist - 1);
        //construct the right branch
        root->right = addNode(inorder, i + 1 , end1, postorder, s2 + dist, end2 - 1);
        return root;
        
    }
    TreeNode *buildTree(vector<int> &inorder, vector<int> &postorder) {
        int inLen = inorder.size();
        int posLen = postorder.size();
        if(inLen == 0 || posLen == 0 || inLen != posLen)
            return NULL;
        TreeNode *root = addNode(inorder, 0, inLen - 1, postorder, 0, posLen - 1);
        return root;
    }