Leetcode 987. Vertical Order Traversal of a Binary Tree

Seawolf

1. Given a binary tree, return the vertical order traversal of its nodes values.
   
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3. For each node at position (X, Y), its left and right children respectively will be at positions (X-1, Y-1) and (X+1, Y-1).
   
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5. Running a vertical line from X = -infinity to X = +infinity, whenever the vertical line touches some nodes, we report the values of the nodes in order from top to bottom (decreasing Y coordinates).
   
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7. If two nodes have the same position, then the value of the node that is reported first is the value that is smaller.
   
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9. Return an list of non-empty reports in order of X coordinate.  Every report will have a list of values of nodes.
   
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13. Example 1:
    
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17. Input: [3,9,20,null,null,15,7]
    
18. Output: [[9],[3,15],[20],[7]]
    
19. Explanation:
    
20. Without loss of generality, we can assume the root node is at position (0, 0):
    
21. Then, the node with value 9 occurs at position (-1, -1);
    
22. The nodes with values 3 and 15 occur at positions (0, 0) and (0, -2);
    
23. The node with value 20 occurs at position (1, -1);
    
24. The node with value 7 occurs at position (2, -2).
    
25. Example 2:
    
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29. Input: [1,2,3,4,5,6,7]
    
30. Output: [[4],[2],[1,5,6],[3],[7]]
    
31. Explanation:
    
32. The node with value 5 and the node with value 6 have the same position according to the given scheme.
    
33. However, in the report "[1,5,6]", the node value of 5 comes first since 5 is smaller than 6.
    
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36. Note:
    
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38. The tree will have between 1 and 1000 nodes.
    
39. Each node's value will be between 0 and 1000.

复制代码

1. # Definition for a binary tree node.
   
2. # class TreeNode:
   
3. #     def __init__(self, val=0, left=None, right=None):
   
4. #         self.val = val
   
5. #         self.left = left
   
6. #         self.right = right
   
7. class Solution:
   
8.     def verticalTraversal(self, root: TreeNode) -> List[List[int]]:
   
9.         res = []
   
10.         if root == None:
    
11.             return res
    
12.         hashmap = collections.defaultdict(list)
    
13.         queue = []
    
14.         cols = []
    
15.         max_val = 0
    
16.         min_val = 0
    
17.         level = 0
    
18.         queue.append(root)
    
19.         cols.append(0)
    
20.         
    
21.         while queue:
    
22.             size = len(queue)
    
23.             for i in range(size):
    
24.                 curt = queue.pop(0)
    
25.                 col = cols.pop(0)
    
26.                 hashmap[col].append((level, curt.val))
    
27.                 if curt.left != None:
    
28.                     min_val = min(min_val, col - 1)
    
29.                     queue.append(curt.left)
    
30.                     cols.append(col - 1)
    
31.                 if curt.right != None:
    
32.                     max_val = max(max_val, col + 1)
    
33.                     queue.append(curt.right)
    
34.                     cols.append(col + 1)
    
35.             level += 1
    
36.         
    
37.         for i in range(min_val, max_val + 1):
    
38.             res.append([j for i, j in sorted(hashmap[i])])
    
39.         return res

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