binary-tree-preorder-traversal.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> solve (TreeNode* root, vector<int>v){
        if (root == NULL)
            return v;
        v.push_back(root -> val);
        v = solve(root -> left, v);
        v = solve(root -> right, v);
        return v;
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int>v;
        v = solve(root, v);
        return v;
    }
};