BFS层次遍历的模板
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2022-07-27 20:58:41
from collections import dequeclass TreeNode: def __init__(self, x): self.val = x self.left = None self.right = Nonedef levelOrder(root: TreeNode) : queue = deque() if root: queue.append(root) while queue:....
层次遍历模板如下
from collections import deque
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def levelOrder(root: TreeNode) :
queue = deque()
if root:
queue.append(root)
while queue:
##每次for循环是一层
for _ in range(0, len(queue)):
root = queue.popleft(0)
if root:
queue.append(root.left)
queue.append(root.right)
答案
先运行工具类,可以从字符串里生成测试用例
from typing import List
import json
from collections import deque
class TreeNode:
def __init__(self, x):
self.val = x
self.left = None
self.right = None
def genTreeFromStr(listStr: str):
list = json.loads(listStr)
if not list:
return None
queue = []
root = TreeNode(list.pop(0))
queue.append(root)
t, i = None, 0
while queue:
if(i >= len(list)):
break
for _ in range(0, len(queue)):
t = queue.pop(0)
if t:
t.left = None if not list[i] else TreeNode(list[i])
i += 1
if(i >= len(list)):
break
t.right = None if not list[i] else TreeNode(list[i])
i += 1
queue.append(t.left)
queue.append(t.right)
return root
def visitTree(root: TreeNode):
queue = []
res = []
queue.append(root)
while(len(queue) != 0):
t = queue.pop(0)
res.append('null' if(t == None) else t.val)
if(t):
queue.append(t.left)
queue.append(t.right)
for i in range(len(res)-1, -1, -1):
if(res[i] == 'null'):
res.pop(i)
else:
break
print(res)
题目
'''
SO32
https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/
层次遍历的话 每次对队列做一个类似快照的操作标记这次遍历是一个层次 for _ in range(0, len(queue)):
之后入队的就是新的一层了
'''
class Solution:
def levelOrder(self, root: TreeNode) -> List[List[int]]:
res = []
queue = []
if root:
queue.append(root)
while queue:
tmp = []
for _ in range(0, len(queue)):
root = queue.pop(0)
if root:
tmp.append(root.val)
queue.append(root.left)
queue.append(root.right)
if tmp:
res.append(tmp)
return res
print(Solution().levelOrder(genTreeFromStr("[3,9,20,null,null,15,7]")))
'''
S107
https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/
SO32 一个题目只是对输出结果进行了反转操作
'''
class Solution:
def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
res = []
queue = deque()
if root:
queue.append(root)
while queue:
tmp = []
for _ in range(0, len(queue)):
root = queue.popleft()
if root:
tmp.append(root.val)
queue.append(root.left)
queue.append(root.right)
if tmp:
res.append(tmp)
res.reverse()
return res
'''
S111
https://leetcode-cn.com/problems/minimum-depth-of-binary-tree/submissions/
做一个层次计数
当一个节点没有左右子节点时返回该计数
'''
class Solution:
def minDepth(self, root: TreeNode) -> int:
queue = deque()
if root:
queue.append(root)
res = 0
while queue:
res += 1
for _ in range(0, len(queue)):
root = queue.popleft()
if root:
if not root.left and not root.right:
return res
queue.append(root.left)
queue.append(root.right)
return res
'''
S559
https://leetcode-cn.com/problems/maximum-depth-of-n-ary-tree/
忽略深度优先遍历
BFS的话思路很简单,做一个层次遍历就OK了
利用元组来记录当前node节点的深度,每次有Children的时候深度+1
dep就是所有深度里的最大深度
'''
class Node:
def __init__(self, val=None, children=None):
self.val = val
self.children = children
class Solution:
def maxDepth(self, root: 'Node') -> int:
queue = deque()
if root:
queue.append((1, root))
dep = 0
while len(queue) != 0:
currentDep, treeNode = queue.popleft()
if treeNode:
dep = max(currentDep, dep)
for t in [] if not treeNode.children else treeNode.children:
queue.append((currentDep+1, t))
return dep
'''
S690
https://leetcode-cn.com/problems/employee-importance/submissions/
其实是一棵多叉树的层次遍历
唯一不同的是树的子儿子不是树本身 而是id
用一个dic把每个id和对应的树存起来就解决了
'''
class Employee:
def __init__(self, id: int, importance: int, subordinates: List[int]):
self.id = id
self.importance = importance
self.subordinates = subordinates
class Solution:
def getImportance(self, employees: List['Employee'], id: int) -> int:
dic = {}
for e in employees:
dic[e.id] = e
if id not in dic:
return 0
root = dic[id]
res = 0
queue = deque()
queue.append(root)
while queue:
for _ in range(0, len(queue)):
root = queue.popleft()
if root:
res += root.importance
if root.subordinates:
for sub in root.subordinates:
queue.append(sub)
return res
'''
S933
https://leetcode-cn.com/problems/cousins-in-binary-tree/
层次遍历 并且保存父节点信息
当某层出现x 或y时直接比对 x y是否都有父节点 并且不相等(没有父节点表示他不在这一层次)
'''
class Solution:
def isCousins(self, root: TreeNode, x: int, y: int) -> bool:
queue = deque()
if root:
queue.append((root, None))
parentX = None
parentY = None
while queue:
for _ in range(0, len(queue)):
root, parent = queue.popleft()
if root:
if root.val == x:
parentX = parent
if root.val == y:
parentY = parent
queue.append((root.left, root.val))
queue.append((root.right, root.val))
if parentX != None or parentY != None:
if not parentX or not parentY:
return False
return parentX != parentY
return False
print(Solution().isCousins(genTreeFromStr("[1,2,3,null,4,null,5]"), 5, 4))
print(Solution().isCousins(genTreeFromStr("[1,2,3,4]"), 3, 4))
'''
[面试题 04.03. 特定深度节点链表](https://leetcode-cn.com/problems/list-of-depth-lcci/)
'''
class ListNode:
def __init__(self, x):
self.val = x
self.next = None
class Solution:
def listOfDepth(self, tree: TreeNode) -> List[ListNode]:
queue = deque()
res=[]
if tree:
queue.append(tree)
while queue:
listNode=ListNode(0)
tmp=listNode
for _ in range(0,len(queue)):
root=queue.popleft()
if root:
tmp.next=ListNode(root.val)
tmp=tmp.next
queue.append(root.left)
queue.append(root.right)
if listNode.next:
res.append(listNode.next)
return res
本文地址:https://blog.csdn.net/qq_21078159/article/details/107100222