SymmetricTree.java

package leetcode;

/**
 * @author eko
 *
 * 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
 * Note:
 * Bonus points if you could solve it both recursively and iteratively.
 *
 */
public class SymmetricTree {
    public static void main(String[] args) {
        TreeNode node = new TreeNode(1);
        node.left = new TreeNode(2);
        node.right = new TreeNode(2);
        node.left.left = new TreeNode(3);
        node.right.right = new TreeNode(3);
        System.out.println(new SymmetricTree().isSymmetric(node));
    }

    public boolean isSymmetric(TreeNode root) {
        return check(root.left, root.right);
    }

    public boolean check(TreeNode left, TreeNode right) {
        if (left == null && right == null) return true;
        if (left == null || right == null) return false;
        if (left.val != right.val) return false;
        return check(left.right, right.left) && check(left.left, right.right);
    }



    public static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;

        TreeNode(int x) {
            val = x;
        }
    }

}