[Leetcode]Binary Tree Preorder Traversal

Given a binary tree, return the preorder traversal of its nodes' values.

简单题，递归和迭代都可以解。

class Solution {
public:
    vector<int> preorderTraversal(TreeNode *root) {
        vector<int> val;
        if(root == NULL)
            return val;
        stack<TreeNode *> path;
        path.push(root);
        while(!path.empty()){
            TreeNode* node = path.top();
            path.pop();
            val.push_back(node->val);

            if(node->right != NULL)
            path.push(node->right);
            if(node->left != NULL)
            path.push(node->left);
        }
        return val;
    }
};