binary_tree_preorder
binary_tree_preorder.cpp 源码
//
// Created by roseduan on 2021/9/20.
// 二叉树的前序遍历
#include "vector"
#include "stack"
using namespace std;
struct TreeNode {
int val;
TreeNode *left;
TreeNode *right;
TreeNode() {}
TreeNode(int val) : val(val) {}
TreeNode(int val, TreeNode *left, TreeNode *right) : val(val), left(left), right(right) {}
};
class Solution {
public:
// 二叉树的前序遍历 递归
vector<int> preorderTraversal1(TreeNode* root) {
vector<int> res;
orderHelper(root, res);
return res;
}
// 迭代
vector<int> preorderTraversal2(TreeNode* root) {
if (!root) {
return {};
}
vector<int> res;
stack<TreeNode*> s;
s.push(root);
while (!s.empty()) {
auto node = s.top();
s.pop();
res.push_back(node->val);
if (node->right) {
s.push(node->right);
}
if (node->left) {
s.push(node->left);
}
}
return res;
}
private:
void orderHelper(TreeNode* root, vector<int> &res) {
if (root == nullptr) {
return;
}
res.push_back(root->val);
orderHelper(root->left, res);
orderHelper(root->right, res);
}
};
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