My answer to https://www.reddit.com/r/cscareerquestions/comments/4evk44/my_google_tech_interviewing_decoded_experience/

longest-consecutive-matrix.js

function Longest(matrix) {
  const cache = getMarkedUpMatrix(matrix);
  
  let current = 1;
  let longest = 1;

  Object.keys(cache).map((value) => {
    const cacheObj = cache[value];
    const cacheObj2 = cache[(parseInt(value, 10) + 1) + ''];
    if (isNeighbor(cacheObj, cacheObj2)) {
      current++;
      
      if (current > longest) {
        longest = current;
      }
    } else {
      current = 1;
    }
  });
  
  return longest;
}

function getMarkedUpMatrix(matrix) {
  const cache = {};
  const markedupMatrix = matrix.map((row, rowIndex) => {
    return row.map((colValue, colIndex) => {
      cache[colValue] = {col: colIndex, row: rowIndex};
    });
  });
  return cache;
}

function isNeighbor(first, second) {
  if (!first || !second) { return false; }

  const x1 = first.row;
  const x2 = first.col;
  const y1 = second.row;
  const y2 = second.col;
  
  const y1range = Boolean([y2 - 1, y2, y2 + 1, y2 + 2].includes(y1));
  const x1range = Boolean([y1 - 1, y1, y1 + 1, y1 + 2].includes(x1));

  return Boolean((x1 === y1 && y1range ) || (x2 === y2 && x1range));
}

console.log(Longest([[0, 1, 4], [2, 3, 5], [8, 7, 6]]));