Leetcode 494 Target Sum

class Solution {
    int ans=0;
    public int findTargetSumWays(int[] nums, int S) {
        if (nums == null || nums.length == 0) return ans;
        dfs(nums, S, 0);
        return ans;
    }
    
    public void dfs(int[] nums, int S,int pos) {
        if(pos==nums.length){
            if(S==0){
                ans++;
            }
            return;
        }
        dfs(nums,S-nums[pos],pos+1);
        dfs(nums,S+nums[pos],pos+1);
    }
}