题目链接/文章讲解/视频讲解：https://programmercarl.com/%E4%BA%8C%E5%8F%89%E6%A0%91%E7%9A%84%E9%80%92%E5%BD%92%E9%81%8D%E5%8E%86.html

CPP

class Solution
{

public:
    void traversal(TreeNode *cur,vector<int>vec){
        if(cur == NULL) return;
        vec.push_back(cur->val);
        traversal(cur->left,vec);
        traversal(cur->right,vec);
    }

    vector<int> preorderTraversal(TreeNode* root){
        vecror<int>res;
        traversal(root,res);
        return res;
    }
};//前序遍历

void traversal(TreeNode* cur, vector<int>& vec) {
    if (cur == NULL) return;
    traversal(cur->left, vec);  // 左
    vec.push_back(cur->val);    // 中
    traversal(cur->right, vec); // 右
}//中序遍历

void traversal(TreeNode* cur, vector<int>& vec) {
    if (cur == NULL) return;
    traversal(cur->left, vec);  // 左
    traversal(cur->right, vec); // 右
    vec.push_back(cur->val);    // 中
}//后序遍历