combination-sum-iv

#include <sstream>

class Solution {
    unordered_map<int, int> dp_map;
    vector<int> vec;

    int func(int target) {
        
        if (dp_map.find(target) != dp_map.end()) {
            return dp_map[target];
        }
        
        int ret = 0;
        for (int i=0; i<vec.size(); ++i) {
            if (vec[i] == target) {
                ret += 1;
            }
            if (vec[i] < target) {
                ret += func(target-vec[i]);
            }
        }
        
        dp_map[target] = ret;
        return ret;
    }

public:

    int combinationSum4(vector<int>& nums, int target) {
        if (nums.size() == 0) {
            return 0;
        }
        vec = nums;
        return func(target);
    }
};

// 以下代码是一开始写的，会将不同排序的组合作为同一种，不符合题意；
// 改过之后是上面的代码。
/*
class Solution {
    unordered_map<string, int> dp_map;
    vector<int> vec;

    int func(int start, int target) {
        stringstream sst;
        sst << start << '_' << target;
        string hstr = sst.str();
        
        if (dp_map.find(hstr) != dp_map.end()) {
            return dp_map[hstr];
        }
        
        int ret = 0;
        int new_target = 0;
        for (int k=0; k*vec[start]<=target; k++) {
            new_target = target - k*vec[start];
            if (new_target == 0) {
                ret += 1;
            }
            if (new_target > 0 && start+1 < vec.size()) {
                ret += func(start+1, new_target);
            }
        }
        dp_map[hstr] = ret;
        return ret;
    }

public:

    int combinationSum4(vector<int>& nums, int target) {
        if (nums.size() == 0) {
            return 0;
        }
        vec = nums;
        return func(start, target);
    }
};
*/