LeetCode - Construct Binary Tree from inorder and postorder Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* buildTree(vector<int>& inorder, vector<int>& postorder) {
        return helper(inorder, 0, inorder.size()-1, postorder, 0, postorder.size()-1);
    }
    TreeNode* helper(vector<int>& inorder, int il, int ir, vector<int>& postorder, int pl, int pr){
        if(il>ir || pl>pr) return NULL;
        int val = postorder[pr];
        TreeNode* cur = new TreeNode(val);
        int cnt = 0;
        for(int i=il;i<ir;i++){
            if(inorder[i]==val) break;
            cnt++;
        }
        cur->left = helper(inorder, il, il+cnt-1, postorder, pl, pl+cnt-1);
        cur->right = helper(inorder, il+cnt+1, ir, postorder, pl+cnt, pr-1);
        return cur;
    }
};