记录了初步解题思路 以及本地实现代码；并不一定为最优 也希望大家能一起探讨 一起进步

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2/12 145. 二叉树的后序遍历

后序遍历：左右根
取值时根右左，结果倒序

class TreeNode(object):def __init__(self, x):self.val = xself.left = Noneself.right = Nonedef postorderTraversal(root):""":type root: TreeNode:rtype: List[int]"""ret =[]stack=[]if not root:return retstack.append(root)while stack:v = stack.pop()if v.left:stack.append(v.left)if v.right:stack.append(v.right)ret.append(v.val)ret.reverse()return ret

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2/13 987. 二叉树的垂序遍历

dfs 记录每一个col下的(row,v)
对col从小到大考虑 每一个col的list tmp
在tmp中根据row,v排序 得到需要的v序列

class TreeNode(object):def __init__(self, val=0, left=None, right=None):self.val = valself.left = leftself.right = rightdef verticalTraversal(root):""":type root: TreeNode:rtype: List[List[int]]"""from collections import defaultdictglobal dicdic = defaultdict(list)def find(node,row,col):global dicif not node:returndic[col].append((row,node.val))find(node.left,row+1,col-1)find(node.right,row+1,col+1)find(root,0,0)l = dic.keys()l.sort()ans = []for i in l:tmp = dic[i]tmp.sort(key=lambda x:(x[0],x[1]))t = [x[1] for x in tmp]ans.append(t)return ans

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2/14 102. 二叉树的层序遍历

BFS 广搜遍历

class TreeNode:def __init__(self, x):self.val = xself.left = Noneself.right = Nonedef levelOrder(root):""":type root: TreeNode:rtype: List[List[int]]"""ret =[]if root==None:return retnodelist = [(root,0)]while len(nodelist)>0:node,level = nodelist[0]if level <= len(ret)-1:res = ret[level]res.append(node.val)ret[level] = reselse:res = [node.val]ret.append(res)nodelist.pop(0)if node.left:nodelist.append((node.left,level+1))if node.right:nodelist.append((node.right,level+1))return ret

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2/15 107. 二叉树的层序遍历 II

BFS 记录层数
将最深的层放在前面

class TreeNode(object):def __init__(self, x):self.val = xself.left = Noneself.right = Nonedef levelOrderBottom(root):""":type root: TreeNode:rtype: List[List[int]]"""if not root:return []l =[]l.append((root,0))tmpl=[]tmp=0ret=[]while len(l)>0:v,level = l.pop(0)if tmp==level:tmpl.append(v.val)else:ret=[tmpl]+rettmpl=[v.val]tmp=levelif v.left:l.append((v.left,level+1))if v.right:l.append((v.right,level+1))ret =[tmpl]+retreturn ret

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2/16 103. 二叉树的锯齿形层序遍历

记录层数 偶数层正序 奇数层逆序

class TreeNode(object):def __init__(self, x):self.val = xself.left = Noneself.right = Nonedef zigzagLevelOrder(root):""":type root: TreeNode:rtype: List[List[int]]"""    if not root:return []l =[]l.append((root,0))tmpl=[]tmp=0ret=[]while len(l)>0:v,level = l.pop(0)if tmp==level:tmpl.append(v.val)else:if level%2==0:tmpl.reverse()ret.append(tmpl)tmpl=[v.val]tmp=levelif v.left:l.append((v.left,level+1))if v.right:l.append((v.right,level+1))if level%2==1:tmpl.reverse()ret.append(tmpl)return ret

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2/17 429. N 叉树的层序遍历

BFS

class Node(object):def __init__(self, val=None, children=None):self.val = valself.children = childrendef levelOrder(root):""":type root: Node:rtype: List[List[int]]"""ans = []if not root:return ansl = [root]while l:tmp = []val = []for node in l:val.append(node.val)tmp.extend(node.children)ans.append(val)l = tmpreturn ans

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2/18 589. N 叉树的前序遍历

1.递归
2.迭代 存入栈中每次取栈顶元素 并将子节点倒序压入栈中

class Node(object):def __init__(self, val=None, children=None):self.val = valself.children = childrendef preorder(root):""":type root: Node:rtype: List[int]"""ans =  []def dfs(node):if not node:return ans.append(node.val)for c in node.children:dfs(c)dfs(root)return ansdef preorder2(root):""":type root: Node:rtype: List[int]"""stack = [root]ans = []while stack:node = stack.pop()if node :ans.append(node.val)for c in node.children[::-1]:stack.append(c)return ans

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