Balance Factor = height(left) - height(right)，必須為 -1, 0, 或 1
LL (Right Rotation)：左子樹的左側過深 → 對失衡點右旋
RR (Left Rotation)：右子樹的右側過深 → 對失衡點左旋
LR (Left-Right)：左子樹的右側過深 → 先左旋左子，再右旋
RL (Right-Left)：右子樹的左側過深 → 先右旋右子，再左旋