One trade: the best profit

def max_profit(prices):
    lowest, best = prices[0], 0
    for p in prices[1:]:
        best = max(best, p - lowest)      # sell today, bought at the cheapest earlier day
        lowest = min(lowest, p)
    return best

#include <algorithm>
#include <vector>

int solve(const std::vector<int>& prices) {
    int lowest = prices[0], best = 0;
    for (int p : prices) {
        best = std::max(best, p - lowest);      // sell today
        lowest = std::min(lowest, p);
    }
    return best;
}

class Solution {
    public int maxProfit(int[] prices) {
        int lowest = prices[0], best = 0;
        for (int p : prices) {
            best = Math.max(best, p - lowest);    // sell today
            lowest = Math.min(lowest, p);
        }
        return best;
    }
}

function maxProfit(prices) {
  let lowest = prices[0], best = 0;
  for (const p of prices) {
    best = Math.max(best, p - lowest);      // sell today
    lowest = Math.min(lowest, p);
  }
  return best;
}

fn solve(prices: &[i64]) -> i64 {
    let (mut lowest, mut best) = (prices[0], 0);
    for &p in prices {
        best = best.max(p - lowest);              // sell today
        lowest = lowest.min(p);
    }
    best
}

solve :: [Int] -> Int
solve prices = maximum (0 : zipWith (-) prices (scanl1 min prices))   -- price minus the lowest so far