1058. Minimize Rounding Error to Meet Target

Given an array of prices [p1,p2...,pn] and a target, round each price pi to Roundi(pi) so that the rounded array [Round1(p1),Round2(p2)...,Roundn(pn)] sums to the given target. Each operation Roundi(pi) could be either Floor(pi) or Ceil(pi).

Return the string "-1" if the rounded array is impossible to sum to target. Otherwise, return the smallest rounding error, which is defined as Σ |Round_i(p_i) - (p_i)| for i from 1 to n , as a string with three places after the decimal.

Example 1:

Input: prices = ["0.700","2.800","4.900"], target = 8
Output: "1.000"
Explanation: 
Use Floor, Ceil and Ceil operations to get (0.7 - 0) + (3 - 2.8) + (5 - 4.9) = 0.7 + 0.2 + 0.1 = 1.0 .

Example 2:

Input: prices = ["1.500","2.500","3.500"], target = 10
Output: "-1"
Explanation: 
It is impossible to meet the target.

Note:

1. 1 <= prices.length <= 500.
2. Each string of prices prices[i] represents a real number which is between 0 and 1000 and has exactly 3 decimal places.
3. target is between 0 and 1000000.