N voracious fish are moving along a river. Calculate how many fish are alive.

fish.swift

func calculateAliveFish(_ A : inout [Int], _ B : inout [Int]) -> Int {
    if A.isEmpty {
        return 0 // base case
    }
    
    var stack: [Int] = [Int]()
    var survivals: Int = 0
    
    for (index,elem) in A.enumerated() {
        if B[index] == 0 {
            while !stack.isEmpty {
                guard let last = stack.last else { break }
                if last < elem {
                    let _ = stack.popLast()
                }else {
                    break
                } 
            }
            
            if stack.isEmpty {
                survivals += 1 // one fish survived in the fight
            }
        }else {
            stack.append(elem)
        }
    }
    
    return survivals + stack.count
}