equilibriumenergy-challenge.js

function checkGameIndex (checkedIndexesCache, list, index) {
  const offset = list[index];

  const is0 = list[index] === 0;
  const is0ToLeft = list[index - offset] === 0;
  const is0ToRight = list[index + offset] === 0;

  if (is0 || is0ToLeft || is0ToRight) {
    return true;
  }

  checkedIndexesCache[index] = true;

  const rightIndex = index + offset;
  const canMoveToRight = rightIndex <= list.length - 1;

  if (
    canMoveToRight &&
    !checkedIndexesCache[rightIndex] &&
    checkGameIndex(checkedIndexesCache, list, rightIndex)
  ) {
    return true;
  }

  const leftIndex = index - offset;
  const canMoveToLeft = leftIndex >= 0;

  if (
    canMoveToLeft &&
    !checkedIndexesCache[leftIndex] &&
    checkGameIndex(checkedIndexesCache, list, leftIndex)
  ) {
    return true;
  }

  return false;
}

function runGame (list, index) {
  if (!list.length) {
    return false;
  }

  // A cache to check for already reviewed indexes.
  // This is to prevent to check on the same indexes more than once.
  const checkedIndexesCache = {};

  return checkGameIndex(checkedIndexesCache, list, index);
}

console.log(0, runGame([], 2) === false);
console.log(1, runGame([4,9,2,100,0], 2) === true);
console.log(2, runGame([0,9], 1) === false);
console.log(3, runGame([1,1,0], 0) === true);
console.log(4, runGame([2,6,3,2,0,1,4], 0) === true);
console.log(5, runGame([2,6,3,2,0,1,4], 3) === true);
console.log(6, runGame([2,6,3,2,0,1,4], 1) === false);
console.log(7, runGame([2,6,3,2,0,2,4], 5) === false);
console.log(8, runGame([1, 4, 1, 1, 9, 0], 2) === true);
console.log(9, runGame([4,9,0,100,0], 2) === true);
console.log(10, runGame([1, 1, 2, 2, 1, 2, 0, 2, 1, 2, 2, 1], 5) === false);