LeetCode-144. Binary Tree Preorder Traversal

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

Example:

Input: ​​[1,null,2,3]​​ 1 \ 2 / 3 Output: ​​[1,2,3]​​

​​题解：​​

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        stack<TreeNode*> q;
        if (root == NULL) {
            return {};
        }
        q.push(root);
        vector<int> res;
        while (q.empty() == false) {
            TreeNode *t = q.top();
            q.pop();
            res.push_back(t->val);
            if (t->right != NULL) {
                q.push(t->right);
            }
            if (t->left != NULL) {
                q.push(t->left);
            }
        }
        return res;
    }
};