思路

除去根节点，每一层添加->val，然后使用前序遍历的顺序

代码

class Solution {
public:
    vector<string> res;
    void getTreePaths(string s, TreeNode* root){
        s += "->";
        s += to_string(root->val);
        if(root->left == nullptr && root->right == nullptr){
             res.push_back(s);
             return;
        }
        
        if(root->left) getTreePaths(s, root->left);
        if(root->right) getTreePaths(s, root->right);

    }

    vector<string> binaryTreePaths(TreeNode* root) {
        if(root->left == nullptr && root->right == nullptr){
            res.push_back(to_string(root->val));
            return res;
        }
        string s;
        if(root->left) getTreePaths(to_string(root->val), root->left);
        if(root->right) getTreePaths(to_string(root->val), root->right);

        return res;
    }
};