268. 丢失的数字

/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function (nums) {
  const n = nums.length
  nums.sort((a, b) => a - b)
  for (let i = 0; i <= n; i++) {
    if (nums[i] !== i) {
      return i
    }
  }
}

/**
 * @param {number[]} nums
 * @return {number}
 */
var missingNumber = function (nums) {
  let len = nums.length
  let addAll = 0
  let i = 0
  while (len > i) {
    addAll = addAll + nums[i]
    i = i + 1
  }
  return ((len + 1) * len) / 2 - addAll
}