Leetcode one

class Solution {
    int dp[21][1001];
    int fun(int i,int n,int target,vector<int>&a)
    {
        //if(csum == target) return 1;
        if(target == 0)
        {
            return 1;
        }
        if(target < 0) return 0;
        if(i == n-1)
        {
           if(target - a[i] == 0) return 1; return 0;
        }

        if(dp[i][target]!=-1) return dp[i][target];
        int op1  = a[i] > 0? fun(i+1,n,target-a[i],a):0;
        int op2  = fun(i+1,n,target,a);
        return dp[i][target] = op1 + op2;
    }
public:
    int findTargetSumWays(vector<int>& a, int target) {
        int n = a.size();
        memset(dp,-1,sizeof dp);
        int sum =  0 , c = 0;
        for(int x:a) {sum += x; if(x == 0) c++;}
        if( (target + sum) & 1) return 0;
        if(target > sum) return 0;
        return fun(0,n,(target+sum)>>1,a) * (1<<c);
    }
};