symmetric tree.java

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