const xSumPairs = (nums, x) => {
  const pairs = [];

  for(let i = 0; i < nums.length; i++) {
    const currNum = nums[i];

    for(let j = i + 1; j < nums.length; j++) {
      if (j === i) continue;

      const nextNum = nums[j];

      if (currNum + nextNum === x) pairs.push([i, j]);
    }
  }

  return pairs;
}

const betterXSumPairs = (nums, x) => {
  const numsAndIndicies = {};
  const pairs = [];

  for(let i = 0; i < nums.length; i++) {
    const currNum = nums[i];
    const missingNum = x - currNum;
    if (numsAndIndicies[missingNum] !== undefined) {
      pairs.push([i, numsAndIndicies[missingNum]])
    } else {
      numsAndIndicies[currNum] = i;
    }
  }

  return pairs;
}