257. Binary Tree Paths

postorder

/**
 * 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<string> binaryTreePaths(TreeNode* root) {
        if (!root) return {};
        auto val = to_string(root->val);
        if (!root->left && !root->right) return {val};
        vector<string> res;
        for (auto &&str : binaryTreePaths(root->left)) {
            res.push_back(val + "->" + str);
        }
        for (auto &&str : binaryTreePaths(root->right)) {
            res.push_back(val + "->" + str);
        }
        return res;
    }
};