Solution101.java

package binary_search_tree_problem;

public class Solution101 {

    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    public boolean isSymmetric(TreeNode root) {
        if (root == null) {
            return true;
        }
        return isSymmetric(root.left, root.right);
    }

    private boolean isSymmetric(TreeNode l1, TreeNode l2) {
        if (l1 == null && l2 == null) {
            return true;
        }
        if (l1 == null || l2 == null) {
            return false;
        }
        if (l1.val != l2.val) {
            return false;
        }
        return isSymmetric(l1.left, l2.right) && isSymmetric(l1.right, l2.left);
    }

}