【手撕】二叉树前序遍历

#include<iostream>
#include<vector>
#include<stack>

using namespace std;

struct TreeNode // 定义树节点的结构
{
    int val;
    TreeNode* left;
    TreeNode* right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

/*
class Solution // 前序遍历二叉树，迭代解法
{
public:
    vector<int> result;
    vector<int> preorderTraversal(TreeNode* root) // 前序遍历二叉树，返回的是数组
    {
        if (root == nullptr)
        {
            return {};
        }
        result.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);

        return result;
    }
};
*/

class Solution //递归解法
{
public:
    vector<int> preorderTraversal(TreeNode* root)
    {
        stack<TreeNode*> sta;
        vector<int> result;
        if (root != nullptr)
        {
            sta.push(root);
        }
        while (!sta.empty())
        {
            int size = sta.size();
            for (int i = 0; i < size; i++)
            {
                TreeNode* node = sta.top();
                sta.pop();
                result.push_back(node->val);
                if (node->right)
                {
                    sta.push(node->right);
                }
                if (node->left)
                {
                    sta.push(node->left);
                }
            }
            
        }
        return result;
    }
};

int main()
{
    TreeNode* node1 = new TreeNode(1);
    TreeNode* node2 = new TreeNode(2);
    TreeNode* node3 = new TreeNode(3);
    node1->right = node2;
    node2->left = node3;
    TreeNode* root = node1;
    
    vector<int> res;
    Solution solution;
    res = solution.preorderTraversal(root);

    vector<int>::iterator iter = res.begin();
    while (iter != res.end())
    {
        cout << *iter << endl;
        iter++;
    }

    return 0;
}