source: https://leetcode.com/problems/symmetric-tree/description/
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
Constraints:
- The number of nodes in the tree is in the range [1, 1000].
- -100 <= Node.val <= 100
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/** * Definition for a binary tree node. * public class TreeNode { * int val; * TreeNode left; * TreeNode right; * TreeNode() {} * TreeNode(int val) { this.val = val; } * TreeNode(int val, TreeNode left, TreeNode right) { * this.val = val; * this.left = left; * this.right = right; * } * } */ class Solution { public boolean isSymmetric(TreeNode root) { return dfs(root, root); } public boolean dfs(TreeNode a, TreeNode b){ if(a==null && b == null) return true; if(a == null || b == null) return false; return a.val == b.val && dfs(a.left, b.right) && dfs(a.right, b.left); } } |
Time complexity:
- O(n), where n is the number of nodes in a tree, as we need to visit every node in the tree
Space Complexity:
- O(1)