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Binary Tree Zigzag Level Order Traversal in Python
Suppose we have a binary tree; we have to find the Zigzag level order traversal. So for the first row, scan from left to right, then right to left from the second row, then again left to right and so on. So if the tree is like −
The traversal sequence will be [[3],[20,9],[15,7]]
To solve this, we will follow these steps −
if the tree is empty, return empty list
queue := make a queue and insert root into the queue, create two empty lists res and res2, and set flag as True
while queue is not empty
make a list of nodes which are present in queue, then insert into res
make a list of node values which are present in queue, then insert into res2
if flag is set, then
i := length of the last sub-list in res – 1
while i >= 0
if ith element of the last sub-list in res has right subtree, then
insert the right subtree into queue
if ith element of the last sub-list in res has left subtree, then
insert the left subtree into queue
decrease i by 1
otherwise
i := length of the last sub-list in res – 1
while i >= 0
if ith element of the last sub-list in res has right subtree, then
insert the right subtree into queue
if ith element of the last sub-list in res has left subtree, then
insert the left subtree into queue
decrease i by 1
return res2
Let us see the following implementation to get better understanding −
Example
class TreeNode: def __init__(self, data, left = None, right = None): self.data = data self.left = left self.right = right def insert(temp,data): que = [] que.append(temp) while (len(que)): temp = que[0] que.pop(0) if (not temp.left): if data is not None: temp.left = TreeNode(data) else: temp.left = TreeNode(0) break else: que.append(temp.left) if (not temp.right): if data is not None: temp.right = TreeNode(data) else: temp.right = TreeNode(0) break else: que.append(temp.right) def make_tree(elements): Tree = TreeNode(elements[0]) for element in elements[1:]: insert(Tree, element) return Tree class Solution(object): def zigzagLevelOrder(self, root): if not root: return [] queue = [root] res = [] res2=[] flag = True while len(queue)!=0: res.append([i for i in queue]) res2.append([i.data for i in queue if i.data != 0]) queue = [] if flag: i=len(res[-1])-1 while i>=0: if res[-1][i].right: queue.append(res[-1][i].right) if res[-1][i].left: queue.append(res[-1][i].left) i-=1 else: i=len(res[-1])-1 while i>=0: if res[-1][i].left: queue.append(res[-1][i].left) if res[-1][i].right: queue.append(res[-1][i].right) i-=1 flag = not flag return res2 ob = Solution() tree = make_tree([3,9,20,None,None,15,7]) print(ob.zigzagLevelOrder(tree))
Input
[3,9,20,null,null,15,7]
Output
[[3], [20, 9], [15, 7]]
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