[LeetCode] 222. Count Complete Tree Nodes

 Problem : https://leetcode.com/problems/count-complete-tree-nodes/

DFS Solution:

Time Complexity = O ( N )

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def countNodes(self, root: TreeNode) -> int:
        if not root:
            return 0
        
        return 1 + self.countNodes(root.left) + self.countNodes(root.right)
        

BFS Solution:

Time Complexity = O ( N )

# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def countNodes(self, root: TreeNode) -> int:
        if not root:
            return 0
        
        queue = deque([root])
        
        level = 0
        result = 0
        
        while queue:
            tmp = deque()
            result += len(queue)
            while queue:
                node = queue.popleft()
                if node.left:
                    tmp.append(node.left)
                if node.right:
                    tmp.append(node.right)
            
            # stop traversal when reach the last level  
            if len(tmp) < 2 ** level:
                return result + len(tmp)
            
            queue = tmp
        return result