LeetCode Binary Tree Inorder Traversal

// 16ms
/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<TreeNode *> s;

        vector <int> v;
        TreeNode *top;
        if(root==NULL)
            return v;

        s.push(root);
        int f=0;
        while(!s.empty())
        {
            top=s.top();
            if(top->left&&f==0)
            {
                s.push(top->left);
            }
            else
            {
                v.push_back(top->val);
                s.pop();
                f=1;
                if(top->right)
                {
                    s.push(top->right);
                    f=0;
                }
            }
        }
        return v;
    }
};