product_of_array_except_self.js

// Given an array nums of n integers where n > 1,  return an array output such that 
// output[i] is equal to the product of all the elements of nums except nums[i].

//   Example:
                 
// Input: [1, 2, 3, 4]   [4, 3, 2, 1]
// Output: [24, 12, 8, 6]
// Constraint: It's guaranteed that the product of the elements of any prefix or 
// suffix of the array (including the whole array) fits in a 32 bit integer.

// Note: Please solve it without division and in O(n).

// Follow up:
// Could you solve it with constant space complexity ? (The output array does not 
//   count as extra space for the purpose of space complexity analysis.)

const productExceptSelf = (arr) => {
  let result = new Array(arr.length),
      runningProduct = 1;

  for (let i = 0; i < arr.length; i++) {
    let num = arr[i];
    result[i] = runningProduct;
    runningProduct *= num;
  }

  runningProduct = 1;
  for (let i = arr.length - 1; i >= 0; i--) {
    let num = arr[i];
    result[i] *= runningProduct;
    runningProduct *= num;
  }

  return result;
};