116 Populating Next Right Pointers in Each Node 填充每个节点的下一个右侧节点指针
Description:
You are given a perfect binary tree where all leaves are on the same level, and every parent has two children. The binary tree has the following definition:
struct Node {
int val;
Node *left;
Node *right;
Node *next;
}
Populate each next pointer to point to its next right node. If there is no next right node, the next pointer should be set to NULL.
Initially, all next pointers are set to NULL.
Follow up:
You may only use constant extra space.
Recursive approach is fine, you may assume implicit stack space does not count as extra space for this problem.
Example:
Example 1:
Input: root = [1,2,3,4,5,6,7]
Output: [1,#,2,3,#,4,5,6,7,#]
Explanation: Given the above perfect binary tree (Figure A), your function should populate each next pointer to point to its next right node, just like in Figure B. The serialized output is in level order as connected by the next pointers, with '#' signifying the end of each level.
Constraints:
The number of nodes in the given tree is less than 4096.
-1000 <= node.val <= 1000
题目描述:
给定一个完美二叉树，其所有叶子节点都在同一层，每个父节点都有两个子节点。二叉树定义如下：
struct Node {
int val;
Node *left;
Node *right;
Node *next;
}
填充它的每个 next 指针，让这个指针指向其下一个右侧节点。如果找不到下一个右侧节点，则将 next 指针设置为 NULL。
初始状态下，所有 next 指针都被设置为 NULL。
示例 :
输入：{"id":"2","left":{"
id":"4","left":null,"next":null,"right":null,"val":5},"val":2},"next":null,"right":{"
id":"6","left":null,"next":null,"right":null,"val":6},"next":null,"right":{"$id":"7","left":null,"next":null,"right":null,"val":7},"val":3},"val":1}
输出：{"id":"2","left":{"
id":"4","left":null,"next":{"
id":"6","left":null,"next":null,"right":null,"val":7},"right":null,"val":6},"right":null,"val":5},"right":null,"val":4},"next":{"
ref":"5"},"next":null,"right":{"
ref":"4"},"val":2},"next":null,"right":{"$ref":"7"},"val":1}
解释：给定二叉树如图 A 所示，你的函数应该填充它的每个 next 指针，以指向其下一个右侧节点，如图 B 所示。
提示：
你只能使用常量级额外空间。
使用递归解题也符合要求，本题中递归程序占用的栈空间不算做额外的空间复杂度。
思路:
- 递归法
左子树的下一个是右子树
如果有 next指针, 则右子树的下一个是 next的左子树 - 迭代法
类似层序遍历, 每次将下一层的 next指针连接好, 设置一个指针指向每一层的第一个节点
每一层遍历时, 按 next指针移动
next指针更新方式与递归法相同
时间复杂度O(n), 空间复杂度O(1)
代码:
C++:
/*
// Definition for a Node.
class Node {
public:
int val;
Node* left;
Node* right;
Node* next;
Node() : val(0), left(NULL), right(NULL), next(NULL) {}
Node(int _val) : val(_val), left(NULL), right(NULL), next(NULL) {}
Node(int _val, Node* _left, Node* _right, Node* _next)
: val(_val), left(_left), right(_right), next(_next) {}
};
*/
class Solution
{
public:
Node* connect(Node* root)
{
if (!root or !root -> left) return root;
root -> left -> next = root -> right;
if (root -> next) root -> right -> next = root -> next -> left;
connect(root -> left);
connect(root -> right);
return root;
}
};
Java:
/*
// Definition for a Node.
class Node {
public int val;
public Node left;
public Node right;
public Node next;
public Node() {}
public Node(int _val) {
val = _val;
}
public Node(int _val, Node _left, Node _right, Node _next) {
val = _val;
left = _left;
right = _right;
next = _next;
}
};
*/
class Solution {
public Node connect(Node root) {
if (root == null) return root;
Node first = root, cur = null;
while (first.left != null) {
cur = first;
while (cur != null) {
cur.left.next = cur.right;
if (cur.next != null) cur.right.next = cur.next.left;
cur = cur.next;
}
first = first.left;
}
return root;
}
}
Python:
"""
# Definition for a Node.
class Node:
def __init__(self, val: int = 0, left: 'Node' = None, right: 'Node' = None, next: 'Node' = None):
self.val = val
self.left = left
self.right = right
self.next = next
"""
class Solution:
def connect(self, root: 'Node') -> 'Node':
if not root or not root.left:
return root
root.left.next = root.right
if root.next:
root.right.next = root.next.left
self.connect(root.left)
self.connect(root.right)
return root
