199. 二叉树的右视图

题目描述

给定一个二叉树的 根节点 root，想象自己站在它的右侧，按照从顶部到底部的顺序，返回从右侧所能看到的节点值。

示例 1：

输入：root = [1,2,3,null,5,null,4]

输出：[1,3,4]

解释：

示例 2：

输入：root = [1,2,3,4,null,null,null,5]

输出：[1,3,4,5]

解释：

示例 3：

输入：root = [1,null,3]

输出：[1,3]

示例 4：

输入：root = []

输出：[]

提示:

二叉树的节点个数的范围是 [0,100]
-100 <= Node.val <= 100

解法

方法一：BFS

我们可以使用广度优先搜索，定义一个队列 \(\textit{q}\)，将根节点放入队列中。每次从队列中取出当前层的所有节点，对于当前节点，我们先判断右子树是否存在，若存在则将右子树放入队列中；再判断左子树是否存在，若存在则将左子树放入队列中。这样每次取出队列中的第一个节点即为该层的右视图节点。

时间复杂度 \(O(n)\)，空间复杂度 \(O(n)\)。其中 \(n\) 为二叉树节点个数。

Python3JavaC++GoTypeScriptRustJavaScript

# 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 rightSideView(self, root: Optional[TreeNode]) -> List[int]:
        ans = []
        if root is None:
            return ans
        q = deque([root])
        while q:
            ans.append(q[0].val)
            for _ in range(len(q)):
                node = q.popleft()
                if node.right:
                    q.append(node.right)
                if node.left:
                    q.append(node.left)
        return ans

/**
 * 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> rightSideView(TreeNode root) {
        List<Integer> ans = new ArrayList<>();
        if (root == null) {
            return ans;
        }
        Deque<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while (!q.isEmpty()) {
            ans.add(q.peekFirst().val);
            for (int k = q.size(); k > 0; --k) {
                TreeNode node = q.poll();
                if (node.right != null) {
                    q.offer(node.right);
                }
                if (node.left != null) {
                    q.offer(node.left);
                }
            }
        }
        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> rightSideView(TreeNode* root) {
        vector<int> ans;
        if (!root) {
            return ans;
        }
        queue<TreeNode*> q{{root}};
        while (q.size()) {
            ans.push_back(q.front()->val);
            for (int k = q.size(); k; --k) {
                auto node = q.front();
                q.pop();
                if (node->right) {
                    q.push(node->right);
                }
                if (node->left) {
                    q.push(node->left);
                }
            }
        }
        return ans;
    }
};

/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func rightSideView(root *TreeNode) (ans []int) {
    if root == nil {
        return
    }
    q := []*TreeNode{root}
    for len(q) > 0 {
        ans = append(ans, q[0].Val)
        for k := len(q); k > 0; k-- {
            node := q[0]
            q = q[1:]
            if node.Right != nil {
                q = append(q, node.Right)
            }
            if node.Left != nil {
                q = append(q, node.Left)
            }
        }
    }
    return
}

/**
 * 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 rightSideView(root: TreeNode | null): number[] {
    const ans: number[] = [];
    if (!root) {
        return ans;
    }
    const q: TreeNode[] = [root];
    while (q.length > 0) {
        ans.push(q[0].val);
        const nq: TreeNode[] = [];
        for (const { left, right } of q) {
            if (right) {
                nq.push(right);
            }
            if (left) {
                nq.push(left);
            }
        }
        q.length = 0;
        q.push(...nq);
    }
    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::cell::RefCell;
use std::collections::VecDeque;
use std::rc::Rc;
impl Solution {
    pub fn right_side_view(root: Option<Rc<RefCell<TreeNode>>>) -> Vec<i32> {
        let mut ans = vec![];
        if root.is_none() {
            return ans;
        }
        let mut q = VecDeque::new();
        q.push_back(root);
        while !q.is_empty() {
            let k = q.len();
            ans.push(q[0].as_ref().unwrap().borrow().val);
            for _ in 0..k {
                if let Some(node) = q.pop_front().unwrap() {
                    let mut node = node.borrow_mut();
                    if node.right.is_some() {
                        q.push_back(node.right.take());
                    }
                    if node.left.is_some() {
                        q.push_back(node.left.take());
                    }
                }
            }
        }
        ans
    }
}

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var rightSideView = function (root) {
    const ans = [];
    if (!root) {
        return ans;
    }
    const q = [root];
    while (q.length > 0) {
        ans.push(q[0].val);
        const nq = [];
        for (const { left, right } of q) {
            if (right) {
                nq.push(right);
            }
            if (left) {
                nq.push(left);
            }
        }
        q.length = 0;
        q.push(...nq);
    }
    return ans;
};

方法二：DFS

使用 DFS 深度优先遍历二叉树，每次先遍历右子树，再遍历左子树，这样每层第一个遍历到的节点即为该层的右视图节点。