Today we are going to do a problem in LeetCode, link to the original problem: finger Offer 32 – Print binary tree II from top to bottom

Topic describes

  • The binary tree is printed from top to bottom layer, with nodes of the same layer printed from left to right, each layer printed to a row.
  • Such as:
    • Given a binary tree: [3,9,20,null,null,15,7],
    3
   / \
  9  20
    /  \
   15   7
Copy the code
  • Returns the result of its hierarchical traversal:
[[3], [9,20], [15,7]Copy the code
  • Tip:
The total number of nodes <= 1000Copy the code

Thought analysis

  • Binary tree traversal using dual-ended queues: collections.deque()
  • Results are saved using an ordered dictionary: collections.ordereddict ()

code

# Python
# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root: return []
        res_dict, queue = collections.OrderedDict(), collections.deque()
        level = 0
        queue.append((root, level))
        while queue:
            node, level = queue.popleft()
            if level in res_dict:
                res_dict[level].append(node.val)
            else:
                res_dict[level] = [node.val]
            level += 1
            if node.left: queue.append((node.left, level))
            if node.right: queue.append((node.right, level))
        
        return res_dict.values()
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conclusion

  • collections.deque()
    • Append () : append function
    • Popleft () : fetch data from the header
  • collections.OrderedDict()
    • Values () : Returns an array of values

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