In a party of N people, only one person known to everyone but doesn't know anyone. Such a person is called celebrity.

The Celebrity Problem

func getCelebrity(input: [[Int]]) -> Int {

    // Add person to stack
    var stack: [Int] = []
    for i in  0..<input.count {
        stack.append(i)
    }
    // pick a person and check with others
    while stack.count > 1 {
        let first = stack.removeLast()
        let second = stack.removeLast()
        if input[first][second] == 1 {
            stack.append(second)
        } else {
            stack.append(first)
        }
    }
    
    let celebrity = stack.removeLast()
    var rowZero = true
    // RowCheck
    for i in 0..<input.count {
        if input[celebrity][i] == 1 {
            rowZero = false
        }
    }
    
    var columnOne = true
    for i in 0..<input.count {
        if input[i][celebrity] == 0 && i != celebrity {
            columnOne = false
        }
    }
    
    if rowZero && columnOne {
        return celebrity
    } else {
        return -1
    }
    
    
}

let input = [[0, 1, 1, 0],
             [0, 0, 1, 0],
             [0, 0, 0, 0],
             [1, 0, 1, 0]]
print(getCelebrity(input: input))