劍指Offer(Python多種思路實現):從上到下打印二叉樹

面試32題：

題目：從上到下打印二叉樹

題：不分行從上到下打印二叉樹

解題思路：

class Solution:
    def PrintFromTopToBottom(self, root):
        # write code here
        if not root:
            return []
        res=[]
        res_val=[]
        res.append(root)
        while len(res)>0:
            node=res.pop(0)
            res_val.append(node.val)
            if node.left:
                res.append(node.left)
            if node.right:
                res.append(node.right)
        return res_val

題目拓展一：分行從上到下打印二叉樹。

題：從上到下按層打印二叉樹，同一層結點從左至右輸出。每一層輸出一行。

解題思路一：

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # 返回二維列表[[1,2],[4,5]]
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        res=[]
        res_val=[]
        res.append(pRoot)
        
        nextLevel=0 #表示下一層節點的數目
        toBePrinted=1 #表示當前層還沒有打印的節點數
        temp=[]
        while len(res)>0:
            node=res[0]
            temp.append(node.val)
            if node.left:
                res.append(node.left)
                nextLevel+=1
                
            if node.right:
                res.append(node.right)
                nextLevel+=1
                
            del res[0]
            toBePrinted-=1
            if toBePrinted==0:
                res_val.append(temp)
                toBePrinted=nextLevel
                nextLevel=0
                temp=[]
        return res_val

解題思路二：

class Solution:
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        res,nodes=[],[pRoot]
        while nodes:
            curStack,nextStack=[],[]
            for node in nodes:
                curStack.append(node.val)
                if node.left:
                    nextStack.append(node.left)
                if node.right:
                    nextStack.append(node.right)
            res.append(curStack)
            nodes=nextStack
        return res

題目拓展二：之字形打印二叉樹

題：請實現一個函數按照之字形打印二叉樹，即第一行按照從左到右的順序打印，第二層按照從右至左的順序打印，第三行按照從左到右的順序打印，其他行以此類推。

解題思路一：

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        res=[]
        nodes=[pRoot]
        right=True
        
        while nodes:
            curStack,nextStack=[],[]
            if right:
                for node in nodes:
                    curStack.append(node.val)
                    if node.left:
                        nextStack.append(node.left)
                    if node.right:
                        nextStack.append(node.right)
            else:
                for node in nodes:
                    curStack.append(node.val)
                    if node.right:
                        nextStack.append(node.right)
                    if node.left:
                        nextStack.append(node.left)
            res.append(curStack)
            nextStack.reverse()
            right=not right
            nodes=nextStack
        return res

解題思路二：

class Solution:
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        res=[]
        nodes=[pRoot]
        leftToRight=True
        
        while nodes:
            curStack,nextStack=[],[]
            for node in nodes:
                curStack.append(node.val)
                if node.left:
                    nextStack.append(node.left)
                if node.right:
                    nextStack.append(node.right)
            if not leftToRight:
                curStack.reverse()
            res.append(curStack)
            leftToRight = not leftToRight
            nodes=nextStack