See: http://careercup.com/question?id=16759664

smallest-range.js

//
// See: http://careercup.com/question?id=16759664
//
// You have k lists of sorted integers. Find the smallest range
// that includes at least one number from each of the k lists.
//
// Input: [[4, 10, 15, 24, 26], [0, 9, 12, 20], [5, 18, 22, 30]]
// Output: [20, 24]
//

const smallestRange = initLists => {
  let k = initLists.length;

  // merge the lists into a single sorted list
  // create data structure with origin & val
  let sortedList = initLists.reduce((acc, curr, i, arr) => {
    return mergeLists(acc, curr.map(val => {
      return {
        origin: i,
        val
      };
    }));
  }, []);

  // find smallest range with unique origins
  let result = [];
  sortedList.reduce((acc, curr, i, arr) => {
    // if any of the current range items are of the same
    // origin, slice the array at i + 1
    for (let i = 0; i < acc.length; i++) {
      if (curr.origin === acc[i].origin) {
        acc = acc.slice(i + 1);
        break;
      }
    }
    acc.push(curr);
    if (acc.length === k) {
      // if this new valid range is less than the
      // smallest range, then save it as the result
      if (result.length === 0 || acc[k - 1].val - acc[0].val < result[1] - result[0]) {
        result = [acc[0].val, acc[k - 1].val];
      }
      // remove the first element from the accumulated array (rotate)
      acc.shift();
    }
    return acc;
  }, []);

  return result;
};

/**
 * Given two sorted int arrays, return a single sorted array
 *
 * @param {array} a - The first list
 * @param {array} b - The second list
 */
const mergeLists = (a, b) => {
  let result = [];
  while (a.length > 0 && b.length > 0) {
    result.push(a[0].val <= b[0].val ? a.shift() : b.shift());
  }
  while (a.length > 0) result.push(a.shift());
  while (b.length > 0) result.push(b.shift());
  return result;
};