a[i]   t = a[low]
       ↑
       |   
       |       ↑
↑      |       |
|      |       |
6      10      7

    a[i] > t

a[i]  a[i+1]   t    
       ↑       ↑    
       |       |     
       |       |       
↑      |       |        
|      |       |  
6     10      10    

a[i] > t 不满足，退出。故而在 a[i+1] 处插入 t

low所在的位置是未排序的位置，左边是已排序区域。[1, length -1]