第三题前缀和，不用动房差公式 def minvar(arr):         x_sum,xx_sum=0,0         prefix_left = [0]*len(arr)         prefix_right = [0]*len(arr)                  for i in range(len(arr)):             x_sum+=arr[i]             xx_sum+=(arr[i]**2)             prefix_left[i] = xx_sum/(i+1.0)-(x_sum/(i+1.0))**2                      x_sum,xx_sum=0,0         for i in range(len(arr)-1,-1,-1):             x_sum+=arr[i]             xx_sum+=(arr[i]**2)             prefix_right[i] = xx_sum/(len(arr)-i)-(x_sum/(len(arr)-i))**2           idx,var = 0,float('inf&(835)#39;)         print(prefix_left)         print(prefix_right)         for i in range(len(arr)-1):                          if prefix_left[i] + prefix_right[i+1]<var:                 idx = i                 var = prefix_left[i] + prefix_right[i+1]         return idx+1