先序遍历

非递归通过压栈来实现遍历

非递归2因为连续压栈再弹栈，所有先压右子树，再压左子树

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
#include <stack>

struct TreeNode {
    int val;
    TreeNode *left;
    TreeNode *right;
    TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
};

// 递归
void PreorderTraversalRecursion(TreeNode* root, std::vector<int>& res)
{
    if (root) {
        res.push_back(root->val);
        PreorderTraversalRecursion(root->left, res);
        PreorderTraversalRecursion(root->right, res);
    }
}

// 非递归1
void PreorderTraversal1(TreeNode* root, std::vector<int>& res)
{
    std::stack<TreeNode*> s;
    while (root || !s.empty()) {
        if (root) {
            res.push_back(root->val);
            s.push(root);
            root = root->left;
        }
        else {
            root = s.top();
            s.pop();
            root = root->right;
        }
    }
}

// 非递归2
void PreorderTraversal2(TreeNode* root, std::vector<int>& res)
{
    std::stack<TreeNode*> s;
    s.push(root);
    while (!s.empty()) {
        root = s.top();
        s.pop();
        if (root) {
            res.push_back(root->val);
            s.push(root->right);
            s.push(root->left);
        }
    }
}


int main()
{
    TreeNode* n1 = new TreeNode(1);
    TreeNode* n2 = new TreeNode(2);
    TreeNode* n3 = new TreeNode(3);
    TreeNode* n4 = new TreeNode(4);
    TreeNode* n5 = new TreeNode(5);
    TreeNode* n6 = new TreeNode(6);
    TreeNode* n7 = new TreeNode(7);
    n1->left = n2;
    n1->right = n5;
    n2->left = n3;
    n2->right = n4;
    n5->left = n6;
    n5->right = n7;

    std::vector<int> res;
    PreorderTraversalRecursion(n1, res);
    std::copy(res.begin(), res.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << std::endl;

    res.clear();
    PreorderTraversal1(n1, res);
    std::copy(res.begin(), res.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << std::endl;

    res.clear();
    PreorderTraversal2(n1, res);
    std::copy(res.begin(), res.end(), std::ostream_iterator<int>(std::cout, " "));
    std::cout << std::endl;
}