苦思苦想 make violent mental efforts; cudgel one's brains; rack one's brains; ransack one's memory for; think hard
苦思冥想 cudgel [rack] one's brains for ideas; deliberate a question time and again; think hard; be lost in deep [hard] thinking; puzzle over; put that in one's pipe and smoke it
忆苦思甜 recall one's sufferings in the old society and contrast them with the happiness in the new; bittersweet memory; call to mind one's past sufferings and think over the source of the present happiness; compare one's happy life in the new society with one's misery in the old; contrast past misery with present happiness; recall one's family's suffering in the old days and tell of one's happy life in the new society; recall past miseries and speak of present happiness; recall past suffering and think over the source of present happiness; recall the past bitterness in contrast to one's present happiness; relate the bitterness of the old society and to savour the sweetness of the new; speak of one's past sufferings and present happy life; tell about one's bitter past and the happy life today