#750 · #2742 · Painting the Walls(给墙壁刷油漆)

You must paint n walls. A paid painter paints wall i in time[i] units and charges cost[i]; while the paid painter is busy, a free painter paints any wall in one time unit at no cost. Return the minimum cost to paint every wall. Constraints: 1 <= n <= 500, 1 <= cost[i] <= 10^6, 1 <= time[i] <= 500.