101. Symmetric Tree

Symmetric Tree Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

1

/ 2 2 / \ / 3 4 4 3

But the following [1,2,2,null,3,null,3] is not:

1

/ 2 2 \ 3 3 python

def isSymmetric(self, root): if not root: return True return self.dfs(root.left, root.right) def dfs(self, l, r): if l and r: return l.val == r.val and self.dfs(l.left, r.right) and self.dfs(l.right, r.left) return l == r

java:

/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode(int x) { val = x; } * } */ class Solution { public boolean isSymmetric(TreeNode root) { if(root == null) return true; return dfs(root.left,root.right); } private boolean dfs(TreeNode l,TreeNode r){ if(l != null && r!= null){ return dfs(l.left,r.right) && dfs(l.right,r.left) && l.val == r.val; } return l == r; } }