题目：https://pintia.cn/problem-sets/994805342720868352/problems/994805364711604224

    主要是DFS函数的递归写法，总结一下吧，框架大概就是下面这种

    1. 结果边界
    2. 查找边界
    3. 下次递归
    1. void DFS(int index, int nowk, int sum,...){
    2.     //结果边界
    3.     if(nowk == k && sum == n){
    4.         if(...){//二级筛选
    6.             ans = temp
    7.         }
    8.     }
    9.     //查找边界
    10.     if(nowk > k || sum > n) return;
    11.     //下次递归
    12.     temp.push_back(index);
    13.     DFS(index, nowk + 1, sum + ..., ...);
    14.     temp.pop();
    15.     DFS(index - 1, nowk, sum, ...)   
    16. }
    18. int main(){
    20. }
    1. #include<vector>
    2. #include<algorithm>
    3. #include<iostream>
    4. #include<cmath>
    5. using namespace std;
    7. vector<int> ans, temp, fac;
    8. int n, k, p, maxFacSum = -1;
    10. void DFS(int index, int nowk, int sum, int facSum){
    11.     if(nowk == k && sum == n){//结果边界
    12.         if(facSum > maxFacSum){
    13.             maxFacSum = facSum;
    14.             ans = temp;
    15.         }
    16.         return;
    17.     }
    18.     if(nowk > k || sum > n) return;
    19.     if(index - 1 >= 0){
    20.         temp.push_back(index);
    21.         DFS(index, nowk + 1, sum + fac[index], facSum + index);//选
    22.         temp.pop_back();
    23.         DFS(index - 1, nowk, sum, facSum);//不选
    24.     }
    25. }
    27. int main(){
    28.     scanf("%d%d%d", &n, &k, &p);    
    29.     for(int i = 0; pow(i * 1.0, p * 1.0) <= n; i++){//初始化fac数组
    30.         fac.push_back(pow(i * 1.0, p * 1.0));
    31.     }
    32.     DFS (fac.size() - 1, 0, 0, 0);
    33.     if(ans.size() == 0){
    34.         printf("Impossible");
    35.         return 0;
    36.     }
    37.     printf("%d =",n);
    38.     for(int i = 0; i < ans.size(); i++){
    39.         printf(" %d^%d", ans[i], p);
    40.         if(i != ans.size() - 1) printf(" +");
    41.     }
    42.     return 0; 
    43. }