给定一个二叉树,返回它的前序遍历

好的，以下是二叉树前序遍历的算法实现，你可以参考一下：

// Definition for a binary tree node.
// struct TreeNode {
//     int val;
//     TreeNode *left;
//     TreeNode *right;
//     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
// };

class Solution {
public:
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> res;
        stack<TreeNode*> s;
        TreeNode* cur = root;
        while (cur != nullptr || !s.empty()) {
            while (cur != nullptr) {
                res.push_back(cur->val);
                s.push(cur);
                cur = cur->left;
            }
            cur = s.top();
            s.pop();
            cur = cur->right;
        }
        return res;
    }
};

希望能对你有所帮助。