void invert(TreeNode x) {
        if (x == null) return;
        TreeNode temp = x.left;
        x.left = x.right;
        x.right = temp;
        invert(x.left);
        invert(x.right);
    }
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
src/problems/easy/SymmetricTree.java