Check if a given binary tree is symmetric.

Examples

    5

  /    \

3        3

/ \ / \

1 4 4 1

is symmetric.

    5

  /    \

3        3

/ \ / \

1 4 1 4

is not symmetric.

Corner Cases

• What if the binary tree is null? Return true in this case.

Solution: 比较left和right，分为同null，其中一个null，同不null(分为val不同，val相同：比较左左和右右，左右和右左)

  public boolean isSymmetric(TreeNode root) {
    if (root == null) {
      return true;
    }
    return check(root.left, root.right);
  }
  private boolean check(TreeNode a, TreeNode b) {
    if (a == null && b == null) {
      return true;
    }
    if (a == null || b == null) {
      return false;
    }
    if (a.key != b.key) {
      return false;
    }
    return check(a.left, b.right) && check(a.right, b.left);
  }