What periods can Lu You's poetry creation be divided into? What are the characteristics of each style?

Lu You is a particularly rich poet with more than 9,300 poems. Scholars often divide his poems into three periods: the first period is before he entered Shu at the age of 46, and there are about 200 existing poems in this period. According to his postscript poems, he had "moved nine tenths" before he was 42 years old, so many early poems were deleted. The second period was from entering Shu to being dismissed at the age of 65 in the sixteenth year (1 189). There are more than 2,400 poems in the past 20 years. During this period, he joined the army in Nanzheng, went deep into the front line of national defense, lived a heroic life full of fighting atmosphere and had a magnificent situation of mountains and rivers.