LeetCode Coin Change 2

class Solution {
public:
    int grid[510][5100];
    int solve(int idx,int amount,vector<int> & coins){
        if(idx == (int) coins.size() or amount == 0){
            return amount == 0;
        }
        if(grid[idx][amount] != -1){
            return grid[idx][amount];
        }
        int res = 0;
        for(int i = 0; i <= amount / coins[idx]; i++){
            res += solve(idx + 1,amount - (coins[idx] * i),coins);
        }
        return grid[idx][amount] = res;
    }

    int change(int amount, vector<int>& coins) {
        memset(grid,-1,sizeof(grid));
        return solve(0,amount,coins);
    }
};