101. Symmetric Tree

Easy

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

Solution:

/**
 * 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) {
        
        if (root == null)
            return true;
        return isSymmetric(root.left, root.right);
        
    }
    
    private boolean isSymmetric(TreeNode a, TreeNode b){
            if(a==null&&b==null){return true;}
            if(a==null||b==null){return false;}
            if(a.val!=b.val){return false;}
            return isSymmetric(a.left,b.right)&&isSymmetric(a.right,b.left);
        }
}