[leetcode] 101. Symmetric Tree _ Algorithm Problem Solve for python

1. Problem

101. Symmetric Tree

Given the root of a binary tree, check whether it is a mirror of itself (i.e., symmetric around its center).

Example 1:

Input: root = [1,2,2,3,4,4,3]
Output: true

Example 2:

Input: root = [1,2,2,null,3,null,3]
Output: false


  • The number of nodes in the tree is in the range [1, 1000].
  • -100 <= Node.val <= 100

2. Solution

I solved this problem like this.

  • Step
    • I solved this problem using bfs.
    • Confirm list value is isSymmetric.
from collections import deque

class Solution:
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        deq = deque([root])
        while deq:
            round_nodes = list(deq)
            deq = deque()

            for i in range(len(round_nodes)//2):
                if round_nodes[i] == None and round_nodes[-(i+1)] == None:
                if round_nodes[i] == None or round_nodes[-(i+1)] == None:
                    return False
                if round_nodes[i].val != round_nodes[-(i+1)].val:
                    return False
            for node in round_nodes:
                if node == None:

            if deq.count(None) == len(deq):

        return True

Other solution is more better. That user solved this problem using dfs.

class Solution:
    def isSymmetric(self, root: Optional[TreeNode]) -> bool:
        if root.left is None and root.right is None:
            return True
        def sym(l,r):
            if l is None and r is None:
                return True
            if l is None or r is None:
                return False
            return l.val == r.val and sym(l.left,r.right) and sym(l.right,r.left)
        return sym(root,root)