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Program to find top view of a binary tree in Python
Suppose we have a binary tree, we have to find the top view of the tree, they will be sorted left−to−right.
So, if the input is like image, then the output will be [3, 5, 8, 6, 9], as 3 is above 2 and 5 is above 7 so they are not visible.
To solve this, we will follow these steps −
view := a new empty map
q := a double ended queue
insert pair (root, 0) at the end of q
start := inf, end := −inf
while q is not empty, do
(node, coord) := left element of q, then remove left element of q
start := minimum of start and coord
end := maximum of end and coord
if coord is not in view, then
view[coord] := value of node
if left of node is not null, then
insert (left of node, coord − 1) at the end of q
if right of node is not null, then
insert (right of node, coord + 1) at the end of q
res := a new list
for i in range start to end, do
if i is in view, then
insert view[i] at the end of res
return res
Let us see the following implementation to get better understanding −
Example
from collections import deque
class TreeNode:
def __init__(self, data, left = None, right = None):
self.val = data
self.left = left
self.right = right
class Solution:
def solve(self, root):
view = {}
q = deque()
q.append((root, 0))
start = float("inf")
end = float("-inf")
while q:
node, coord = q.popleft()
start = min(start, coord)
end = max(end, coord)
if coord not in view:
view[coord] = node.val
if node.left:
q.append((node.left, coord - 1))
if node.right:
q.append((node.right, coord + 1))
res = []
for i in range(start, end + 1):
if i in view:
res.append(view[i])
return res
ob = Solution()
root = TreeNode(5)
root.left = TreeNode(3)
root.right = TreeNode(8)
root.right.left = TreeNode(7)
root.right.right = TreeNode(6)
root.right.left.left = TreeNode(2)
root.right.right.right = TreeNode(9)
print(ob.solve(root))Input
root = TreeNode(5) root.left = TreeNode(3) root.right = TreeNode(8) root.right.left = TreeNode(7) root.right.right = TreeNode(6) root.right.left.left = TreeNode(2) root.right.right.right = TreeNode(9)
Output
[3, 5, 8, 6, 9]
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