[LeetCode]Binary Tree Preorder Traversal

题目描述:(链接）

Given a binary tree, return the preorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1
    
     2
    /
   3

return [1,2,3].

Note: Recursive solution is trivial, could you do it iteratively?

解题思路：

迭代版：

/**
 * 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<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        stack<TreeNode *> cache;
        TreeNode *p = root;
        if (p != nullptr) {
            cache.push(p);
        }
        
        while (!cache.empty()) {
            p = cache.top();
            cache.pop();
            result.push_back(p->val);
            
            if (p->right != nullptr) {
                cache.push(p->right);
            }
            
            if (p->left != nullptr) {
                cache.push(p->left);
            }
        }
        
        return result;
    }
};

 递归版：

/**
 * 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<int> preorderTraversal(TreeNode* root) {
        if (root != nullptr) {
            result.push_back(root->val);
            preorderTraversal(root->left);
            preorderTraversal(root->right);
        }
        
        return result;
    }
private:
    vector<int> result;
};