5kyu_LastDigitOfALargeNumber.js

// 5kyu - Last Digit of a Large Number 

// Define a function that takes in two non-negative integers aaa and bbb and returns the last decimal digit of aba^ba 
// b
//  . Note that aaa and bbb may be very large!

// For example, the last decimal digit of 979^79 
// 7
//   is 999, since 97=47829699^7 = 47829699 
// 7
//  =4782969. The last decimal digit of (2200)2300({2^{200}})^{2^{300}}(2 
// 200
//  ) 
// 2 
// 300
 
//  , which has over 109210^{92}10 
// 92
//   decimal digits, is 666. Also, please take 000^00 
// 0
//   to be 111.

// You may assume that the input will always be valid.

// Examples
// lastDigit("4", "1")            // returns 4
// lastDigit("4", "2")            // returns 6
// lastDigit("9", "7")            // returns 9    
// lastDigit("10","10000000000")  // returns 0
// Remarks
// JavaScript, C++, R, PureScript
// Since these languages don't have native arbitrarily large integers, your arguments are going to be strings representing non-negative integers instead.

var lastDigit = function(str1, str2){  
  return +!+str2 || Math.pow(str1.slice(-1) % 10, str2.slice(-2) % 4 || 4) % 10;
}