Leetcode[144]-Binary Tree Preorder Traversal

Link: https://leetcode.com/problems/binary-tree-preorder-traversal/

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].

C++递归遍历：

/**C++ * 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> nums;
    vector<int> preorderTraversal(TreeNode* root) {
        if(root == NULL) return nums;
        preOrder(root);
        return nums;
    }
    void preOrder(TreeNode * root){
        if(root == NULL) return;
        nums.push_back(root->val);
        if(root->left)  preOrder(root->left);
        if(root->right) preOrder(root->right);
    }   
};

非递归遍历C++：

vector<int> preorderTraversal(TreeNode* root) {
    vector<int > nums;
    stack<TreeNode* > stk;
    if(root == NULL) return nums;
    TreeNode * p = root;
    stk.push(p);

    while(!stk.empty()){
        p = stk.top();
        nums.push_back(p->val);
        stk.pop();
        if(p->right) stk.push(p->right);
        if(p->left) stk.push(p->left);
    }
}