LeetCode 41 First Missing Positive

class Solution {
public:
    int firstMissingPositive(vector<int>& nums) {
        
         int len = nums.size()+1;
         int a[len];
        memset(a,0,sizeof(a));
         for(int i=0;i<nums.size();i++)
         {
             if(nums[i]>=len||nums[i]<=0)
                 continue;
             
             
             a[nums[i]]=1;
         }
        
        for(int i=1;i<len;i++)
        {
            if(a[i]!=1)
                return i;
        }
        return len;
    }
};