Cacher is the code snippet organizer for pro developers

//递归法：
/**
 * 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:
    void pTra(TreeNode* root, vector<int> &vec) {
        if (root == NULL) return;
        vec.insert(vec.end(), root->val);
        pTra(root->left, vec);
        pTra(root->right, vec);
    }
    vector<int> preorderTraversal(TreeNode* root) {
        vector<int> result;
        pTra(root, result);
        return result;
    }
};

//非递归法：
/**
 * 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 *> rootStack;
        rootStack.push(root);
        TreeNode* curNode;
        while(!rootStack.empty()) {
            curNode = rootStack.top();
            rootStack.pop();
            if (curNode == NULL) 
                continue;
            result.insert(result.end(), curNode->val);
            rootStack.push(curNode->right);
            rootStack.push(curNode->left);
        }
        return result;
    }
};