iteration Archives - Huahua's Tech Road

Given a binary tree, return the preorder traversal of its nodes’ values.

Example:

Input: [1,null,2,3]
   1
    \
     2
    /
   3

Output: [1,2,3]

Follow up: Recursive solution is trivial, could you do it iteratively?

Solution 1: Recursion

Time complexity: O(n)
Space complexity: O(n)

// Author: Huahua

class Solution {

public:

vector<int> preorderTraversal(TreeNode* root) {

vector<int> ans;

function<void(TreeNode*)> preorder = [&](TreeNode* n) {

if (!n) return;

ans.push_back(n->val);

preorder(n->left);

preorder(n->right);

};

preorder(root);

return ans;

}

};

Time complexity: O(n)
Space complexity: O(n)

// Author: Huahua

class Solution {

public:

vector<int> preorderTraversal(TreeNode* root) {

vector<int> ans;

stack<TreeNode*> s;

if (root) s.push(root);

while (!s.empty()) {

TreeNode* n = s.top();

ans.push_back(n->val);

s.pop();

if (n->right) s.push(n->right);

if (n->left) s.push(n->left);

}

return ans;

}