static int twoStack(int x, int[] a, int[] b) {
        int sum = 0, maxBucket=0;
        int bj = 0, ai = 0;

        while(ai < a.length  && (sum+a[ai]) <= x ){
            sum+=a[ai++];
            maxBucket++; // add max buckets from 1st stack , sum <=x
        }

        while(bj < b.length){ // add each element from b stack
            sum+=b[bj++];

            while(sum>x && ai > 0){// while sum is larger
                sum-=a[--ai]; // keep removing element from a stack from top
            }

            if(sum<=x && ai+bj>maxBucket){// if ai+aj > maxBucket
                maxBucket= ai+bj; // maxbucket is ai+aj
            }
        }

        return maxBucket;
    }