144. Binary Tree Preorder Traversal

Description

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

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]


Example 2:

Input: root = []
Output: []


Example 3:

Input: root = [1]
Output: [1]


Constraints:

• The number of nodes in the tree is in the range [0, 100].
• -100 <= Node.val <= 100

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

Solutions

1. Recusive Traversal

2. Non-recursive using Stack

3. Morris Traversal

• /**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode() {}
*     TreeNode(int val) { this.val = val; }
*     TreeNode(int val, TreeNode left, TreeNode right) {
*         this.val = val;
*         this.left = left;
*         this.right = right;
*     }
* }
*/
class Solution {
public List<Integer> preorderTraversal(TreeNode root) {
List<Integer> ans = new ArrayList<>();
while (root != null) {
if (root.left == null) {
root = root.right;
} else {
TreeNode prev = root.left;
while (prev.right != null && prev.right != root) {
prev = prev.right;
}
if (prev.right == null) {
prev.right = root;
root = root.left;
} else {
prev.right = null;
root = root.right;
}
}
}
return ans;
}
}

• /**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode() : val(0), left(nullptr), right(nullptr) {}
*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
* };
*/
class Solution {
public:
vector<int> preorderTraversal(TreeNode* root) {
vector<int> ans;
while (root) {
if (!root->left) {
ans.push_back(root->val);
root = root->right;
} else {
TreeNode* prev = root->left;
while (prev->right && prev->right != root) {
prev = prev->right;
}
if (!prev->right) {
ans.push_back(root->val);
prev->right = root;
root = root->left;
} else {
prev->right = nullptr;
root = root->right;
}
}
}
return ans;
}
};

• # Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
def preorderTraversal(self, root: Optional[TreeNode]) -> List[int]:
ans = []
while root:
if root.left is None:
ans.append(root.val)
root = root.right
else:
prev = root.left
while prev.right and prev.right != root:
prev = prev.right
if prev.right is None:
ans.append(root.val)
prev.right = root
root = root.left
else:
prev.right = None
root = root.right
return ans


• /**
* Definition for a binary tree node.
* type TreeNode struct {
*     Val int
*     Left *TreeNode
*     Right *TreeNode
* }
*/
func preorderTraversal(root *TreeNode) []int {
var ans []int
for root != nil {
if root.Left == nil {
ans = append(ans, root.Val)
root = root.Right
} else {
prev := root.Left
for prev.Right != nil && prev.Right != root {
prev = prev.Right
}
if prev.Right == nil {
ans = append(ans, root.Val)
prev.Right = root
root = root.Left
} else {
prev.Right = nil
root = root.Right
}
}
}
return ans
}

• /**
* Definition for a binary tree node.
* class TreeNode {
*     val: number
*     left: TreeNode | null
*     right: TreeNode | null
*     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
*         this.val = (val===undefined ? 0 : val)
*         this.left = (left===undefined ? null : left)
*         this.right = (right===undefined ? null : right)
*     }
* }
*/

function preorderTraversal(root: TreeNode | null): number[] {
let ans = [];
while (root) {
if (!root.left) {
ans.push(root.val);
root = root.right;
} else {
let prev = root.left;
while (prev.right && prev.right != root) {
prev = prev.right;
}
if (!prev.right) {
ans.push(root.val);
prev.right = root;
root = root.left;
} else {
prev.right = null;
root = root.right;
}
}
}
return ans;
}


• // Definition for a binary tree node.
// #[derive(Debug, PartialEq, Eq)]
// pub struct TreeNode {
//   pub val: i32,
//   pub left: Option<Rc<RefCell<TreeNode>>>,
//   pub right: Option<Rc<RefCell<TreeNode>>>,
// }
//
// impl TreeNode {
//   #[inline]
//   pub fn new(val: i32) -> Self {
//     TreeNode {
//       val,
//       left: None,
//       right: None
//     }
//   }
// }
use std::rc::Rc;
use std::cell::RefCell;
impl Solution {
fn dfs(root: &Option<Rc<RefCell<TreeNode>>>, res: &mut Vec<i32>) {
if root.is_none() {
return;
}
let node = root.as_ref().unwrap().borrow();
res.push(node.val);
Self::dfs(&node.left, res);
Self::dfs(&node.right, res);
}

pub fn preorder_traversal(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
let mut res = vec![];
Self::dfs(&root, &mut res);
res
}
}