DFS and part of the problem

Given integers a1, a2, ......, an, several judges whether to choose the number, and to make them exactly to be k.

limitation factor

1<=n<=20

-10^n<=ai<=10^8

-10^8<=k<=10^8

Input:

n=4

a={1,2,4,7}

k=13

Output:

Yes (13 = 2 + 4 + 7)

int a[MAX_N];
int n, k;

// 已经从i项得到了和sum， 然后对于i项之后的进行分支
bool dfs(int i, int sum)
{
    if(i == n) return sum == k;
    
    // 不加上a[i]的情况
    if(dfs(i + 1, sum)) return true;

    // 加上a[i]的情况
    if(dfs(i + 1, sum + a[i]) return true;

    return false;
}

void solve()
{
    if(dfs(0, 0)) printf("Yes\n");
    else printf("No\n"); 
}