LeetCode：144_Binary Tree Preorder Traversal | 二叉树的前序遍历 | Medium

题目：Binary Tree Preorder Traversal

二叉树的前序遍历，同样使用栈来解，代码如下：

struct TreeNode {
    int            val;
    TreeNode*    left;
    TreeNode*    right;
    TreeNode(int x): val(x), left(NULL),right(NULL) {}
};

vector<int> preorderTraversal(TreeNode *root) //非递归的前序遍历(用栈实现)
{
    if (NULL == root) {
        cout << "tree is empty!" << endl;
        exit(0);
    }

    TreeNode *pre = root;
    //TreeNode *pTemp = pre;
    stack<TreeNode *> tree_stack;
    vector<int> tree_vector;
    
    tree_stack.push(pre);
    while (!tree_stack.empty()) {
        TreeNode *pTemp = tree_stack.top();
        tree_stack.pop();

        tree_vector.push_back(pTemp->val);
        if (pTemp->right != NULL) 
            tree_stack.push(pTemp->right);
        if (pTemp->left != NULL)
            tree_stack.push(pTemp->left);
    }
    return tree_vector;
}