[LeetCode] 116&117. Populating Next Right Pointers in Each Node I&II_Medium tag: BFS(Dont know why leetcode tag it as DFS...)
2018-07-13 07:33 Johnson_强生仔仔 阅读(218) 评论(0) 编辑 收藏 举报Given a binary tree
struct TreeLinkNode { TreeLinkNode *left; TreeLinkNode *right; TreeLinkNode *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
.
Note:
- You may only use constant extra space.
- Recursive approach is fine, implicit stack space does not count as extra space for this problem.
Example:
Given the following binary tree,
1 / \ 2 3 / \ \ 4 5 7
After calling your function, the tree should look like:
1 -> NULL / \ 2 -> 3 -> NULL / \ \ 4-> 5 -> 7 -> NULL
这个题目因为是每一层之间的元素的关系, 所以很明显用BFS? 不知道为啥tag DFS, anyways, 我想的思路为, 利用每一层heig不一样, 然后设置一个pre, pre_h, 如果pre_h跟现在的heig相同, 那
表明是同一层, pre.next = node, 然后BFS即可.
12/03/2019 Update: 可以用Space:O(1), 设置start 和cur对每一层的来遍历,因为是perfect binary tree,所以如果有left child,肯定有right child。
1. Constraints
1) can be empty
2. Ideas
BFS T: O(n) S: O(n)
按层遍历 T: O(n) S: O(1)
3. Code
1)
1 class Solution: 2 def connect(self, root): 3 pre, pre_h, queue = None, -1, collections.deque([(root, 0)]) 4 while queue: 5 node, heig = queue.popleft() 6 if node: 7 if pre_h == heig: 8 pre.next = node 9 pre, pre_h = node, heig 10 queue.append(node.left) 11 queue.append(node.right)
2) S: O(1) for 116, perfect binary tree
class Solution: def connect(self, root: 'Node') -> 'Node': start = root while start: cur = start while cur: if cur.left: cur.left.next = cur.right if cur.next: cur.right.next = cur.next.left cur = cur.next start = start.left return root
3) S: O(1) for 117, general binary tree, 用dummy来记录每一层之前的点, cur分别是一层中当时的点, root则为已经有next的parent的那一层的cur node.
""" # 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': dummy, start = Node(), root pre = dummy while start: cur = start while cur: if cur.left: pre.next = cur.left pre = cur.left if cur.right: pre.next = cur.right pre = cur.right cur = cur.next start = dummy.next # get the next start pre = dummy # get the dummy pre.next = None # remove the pointer from dummy return root
4. Test cases
1 / \ 2 3 / \ \ 4 5 7