【LeetCode 144_二叉树_遍历】Binary Tree Preorder Traversal

解法一：非递归

vector<int> preorderTraversal(TreeNode* root) 
{
    vector<int> res;
    if (root == NULL)
        return res;

    stack<TreeNode*> node_stack;
    TreeNode *p = root;
    while (p || !node_stack.empty()) {
        if (p) {
            res.push_back(p->val);
            node_stack.push(p);
            p = p->left;
        } else {
            p = node_stack.top();
            node_stack.pop();
            p = p->right;
        }
    }
    return res;
}

解法二：递归

void preorderTraversalSub(TreeNode* root, vector<int> &res)
{
    if (root == NULL)
        return;
    res.push_back(root->val);
    preorderTraversalSub(root->left, res);
    preorderTraversalSub(root->right, res);
}

vector<int> preorderTraversal(TreeNode* root)
{
    vector<int> res;
    preorderTraversalSub(root, res);
    return res;
}