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;
}