Leetcode 1449，动态规划

如何解决Leetcode 1449，动态规划

https://leetcode.com/problems/form-largest-integer-with-digits-that-add-up-to-target/

我已经为这个问题编写了递归方法，但我不知道如何记住这个递归。比如应该用什么数据结构来记忆

class Solution {
    private String res = "";
    public String largestNumber(int[] cost,int target) {
        find(cost,target,"");
        if(res == "") return "0";
        return res;
    }
    public void find(int[] costs,int target,String ssf) {
        if(target < 0) return;         
        if(target == 0) {
            if(ssf.length() > res.length()) {
                res = ssf;
            }
            return;
        }
        for(int i=costs.length - 1; i>=0; i--) {
            find(costs,target - costs[i],ssf + (i+1));
        }
    }

}