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//! Binary Tree Level Order Traversal[leetcode: binary_tree_level_order_traversal](https://leetcode.com/problems/binary-tree-level-order-traversal/) //! //! Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level). //! //! For example: //! Given binary tree `[3,9,20,null,null,15,7]`, //! //! ``` //! 3 //! / \ //! 9 20 //! / \ //! 15 7 //! ``` //! return its level order traversal as: //! ``` //! [ //! [3], //! [9,20], //! [15,7] //! ] //! ``` use std::rc::Rc; use std::cell::RefCell; use std::collections::VecDeque; /// # Solutions /// /// # Approach 1: BFS /// /// * Time complexity: O(n) /// /// * Space complexity: O(n) /// /// * Runtime: 0 ms /// /// * Memory: 2.6 MB /// /// ```rust /// // 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; /// use std::collections::VecDeque; /// impl Solution { /// pub fn level_order(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<Vec<i32>> { /// let mut result: Vec<Vec<i32>> = vec![]; /// if root.is_none() { return result; } /// /// let mut deque: VecDeque<Option<Rc<RefCell<TreeNode>>>> = VecDeque::new(); /// deque.push_back(root); /// /// while !deque.is_empty() { /// let mut current_level = vec![]; /// let mut added = false; /// let level_size = deque.len(); /// /// for _i in 0..level_size { /// let n = deque.pop_front(); /// if let Some(Some(node)) = n { /// current_level.push(node.borrow().val); /// added = true; /// if !node.borrow().left.is_none() { deque.push_back(node.borrow().left.clone()); } /// if !node.borrow().right.is_none() { deque.push_back(node.borrow().right.clone()); } /// } /// } /// /// if !added { break; } /// /// result.push(current_level); /// } /// /// result /// } /// } /// ``` /// /// # Approach 2: DFS /// /// * Time complexity: O(n) /// /// * Space complexity: O(n) /// /// * Runtime: 0 ms /// /// * Memory: 2.7 MB /// /// ```rust /// // 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 { /// pub fn level_order(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<Vec<i32>> { /// let mut result: Vec<Vec<i32>> = vec![]; /// Self::_dfs(&mut result, root, 0); /// result /// } /// /// pub fn _dfs(result: &mut Vec<Vec<i32>>, root: Option<Rc<RefCell<TreeNode>>>, level: usize) { /// if let Some(node) = root { /// if result.len() == level { /// result.push(vec![node.borrow().val]); /// } else { /// result[level].push(node.borrow().val); /// } /// Self::_dfs(result, node.borrow().left.clone(), level + 1); /// Self::_dfs(result, node.borrow().right.clone(), level + 1) /// } /// } /// } /// ``` /// pub fn level_order(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<Vec<i32>> { let mut result: Vec<Vec<i32>> = vec![]; if root.is_none() { return result; } let mut deque: VecDeque<Option<Rc<RefCell<TreeNode>>>> = VecDeque::new(); deque.push_back(root); while !deque.is_empty() { let mut current_level = vec![]; let mut added = false; let level_size = deque.len(); for _i in 0..level_size { let n = deque.pop_front(); if let Some(Some(node)) = n { current_level.push(node.borrow().val); added = true; if !node.borrow().left.is_none() { deque.push_back(node.borrow().left.clone()); } if !node.borrow().right.is_none() { deque.push_back(node.borrow().right.clone()); } } } if !added { break; } result.push(current_level); } result } // 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 } } }