2 questions in 35 min optimised code for both is expected

1. Similar to valid parenthesis but balance the string and return
2. Given 2 arrays of cost of departure and return [1,4,6,7,8] and [2,5,10,8,6] find the min cost of round travel. Same day round travel can be possible. Here answer would be 3

Was able to solve both, bust wasted lot of time on first. Wasnt able to give optimised solution for second. Result - Reject