No101.symmetric-tree.cs

/*
 * Difficulty:
 * Easy
 *
 * Desc:
 * 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
 *
 * Follow up: Solve it both recursively and iteratively.
*/

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public bool IsSymmetric(TreeNode root) {
        if (root == null) return true;

        bool 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.left, right.right) && Check(left.right, right.left);
        }

        return Check(root.left, root.right);
    }
}