LeetCode-Binary Tree Preorder Traversal[二叉树]

#include<iostream>
#include<vector>
#include<stack>
using namespace std;

class Solution
{
public:
    vector<int> preorderTraversal(TreeNode *root)
    {
        vector<int> result;
        const TreeNode *p;
        stack<const TreeNode *> s;

        p = root;
        if(p != nullptr)    s.push(p);
        while(!s.empty())
        {
            p = s.top();
            s.pop();
            result.push_back(p->val);

            if(p->right != nullptr)    s.push(p->right);
            if(p->left != nullptr)    s.push(p->left);    //stack 后进先出 所以先判断right
        }
        return result;
    }
}