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| 1 | +/* |
| 2 | + * Problem: 144 |
| 3 | + * Name: Binary Tree Preorder Traversal |
| 4 | + * Difficulty: Easy |
| 5 | + * Topic: Binary Trees |
| 6 | + * Link: https://leetcode.com/problems/binary-tree-preorder-traversal |
| 7 | + */ |
| 8 | + |
| 9 | +#include <bits/stdc++.h> |
| 10 | +#include <cstddef> |
| 11 | +using namespace std; |
| 12 | + |
| 13 | +// Tree Node Implementation |
| 14 | +struct TreeNode { |
| 15 | + int val; |
| 16 | + TreeNode *left; |
| 17 | + TreeNode *right; |
| 18 | + TreeNode() : val(0), left(nullptr), right(nullptr) {} |
| 19 | + TreeNode(int x) : val(x), left(nullptr), right(nullptr) {} |
| 20 | + TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {} |
| 21 | +}; |
| 22 | + |
| 23 | +// Stack |
| 24 | +// Time Complexity: O(n) |
| 25 | +// Space Complexity: O(n) |
| 26 | +vector<int> preorderTraversal(TreeNode* root) { |
| 27 | + if (root == nullptr) {return {};} |
| 28 | + vector<int> result; |
| 29 | + stack<TreeNode*> remaining; |
| 30 | + remaining.push(root); |
| 31 | + |
| 32 | + while (!remaining.empty()) { |
| 33 | + TreeNode *current = remaining.top(); |
| 34 | + remaining.pop(); |
| 35 | + result.push_back(current->val); |
| 36 | + |
| 37 | + if (current->right != nullptr) {remaining.push(current->right);} |
| 38 | + if (current->left != nullptr) {remaining.push(current->left);} |
| 39 | + } |
| 40 | + return result; |
| 41 | +} |
| 42 | + |
| 43 | +// Call Stack |
| 44 | +// Time Complexity: O(n) |
| 45 | +// Space Complexity: O(n) |
| 46 | +void preorderHelper(TreeNode* node, vector<int>& result); |
| 47 | +vector<int> preorderTraversal(TreeNode* root) { |
| 48 | + vector<int> result; |
| 49 | + preorderHelper(root, result); |
| 50 | + return result; |
| 51 | +} |
| 52 | +void preorderHelper(TreeNode* node, vector<int>& result){ |
| 53 | + if (node == nullptr) {return;} |
| 54 | + result.push_back(node->val), |
| 55 | + preorderHelper(node->left, result); |
| 56 | + preorderHelper(node->right, result); |
| 57 | +} |
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