leetcode 101. Symmetric Tree

Seawolf

1. Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
   
2. 
   
3. For example, this binary tree [1,2,2,3,4,4,3] is symmetric:
   
4. 
   
5.     1
   
6.    / \
   
7.   2   2
   
8. / \ / \
   
9. 3  4 4  3
   
10. 
    
11. 
    
12. But the following [1,2,2,null,3,null,3] is not:
    
13. 
    
14.     1
    
15.    / \
    
16.   2   2
    
17.    \   \
    
18.    3    3
    
19. 
    
20. 
    
21. Note:
    
22. Bonus points if you could solve it both recursively and iteratively.

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1. /**
   
2. * Definition for a binary tree node.
   
3. * public class TreeNode {
   
4. *     int val;
   
5. *     TreeNode left;
   
6. *     TreeNode right;
   
7. *     TreeNode(int x) { val = x; }
   
8. * }
   
9. */
   
10. class Solution {
    
11.    
    
12.     public boolean isSymmetric(TreeNode root) {
    
13.         if(root == null) return true;
    
14.         
    
15.         return helper(root.left, root. right) ;
    
16.     }
    
17.    
    
18.     public boolean helper(TreeNode left, TreeNode right){
    
19.         
    
20.         if(left == null && right == null) return true;
    
21.         
    
22.         if(left == null || right == null) return false;
    
23.         
    
24.         if(left.val != right.val)
    
25.             return false;
    
26.         
    
27.         return helper(left.left,right.right) && helper(left.right, right.left);
    
28.         
    
29.     }
    
30. }

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