101. 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
*****/

//Java solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymm(TreeNode left, TreeNode right){
        if(left == null && right == null){return true;}
        
        if(left == null || right == null){
            return false;
        }
        
        if(left.val == right.val){
            return isSymm(left.left, right.right) && isSymm(left.right, right.left);
        }else{return false;}
    }
    
    public boolean isSymmetric(TreeNode root) {
        if(root == null) {return true;}
        
        return isSymm(root.left, root.right);
    }
}