3月3日(3) Binary Tree Preorder Traversal

/**
 * Definition for binary tree
 * 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> r;
        if (root == NULL) return r;
        
        r.push_back(root->val);
         
        if (root->left != NULL) 
        {
            vector<int> v = preorderTraversal(root->left);
            r.reserve(r.size() + distance(v.begin(),v.end()));
            r.insert(r.end(),v.begin(),v.end());
        }
        
        if (root->right != NULL) 
        {
            vector<int> v = preorderTraversal(root->right);
            r.reserve(r.size() + distance(v.begin(),v.end()));
            r.insert(r.end(),v.begin(),v.end());
        }
        return r;
    }
};