163. Missing Ranges

给定上下限，确定有序数组中缺失的区间。

• 时间复杂度O(N)，空间复杂度O(1)
• Runtime: 76 ms, faster than 35.71%
• Memory Usage: 38.7 MB, less than 61.90%

/**
 * @param {number[]} nums
 * @param {number} lower
 * @param {number} upper
 * @return {string[]}
 */
var findMissingRanges = function(nums, lower, upper) {
  let res = []
  let prev = lower - 1;
  for (let i = 0; i <= nums.length; i++) {
    let cur = (i < nums.length ? nums[i] : upper + 1);
    if(prev + 1 <= cur - 1) {
      res.push(formatRange(prev + 1, cur - 1));
    }
    prev = cur;
  }
  return res;
};


var formatRange = function (lower, upper) {
  if (lower === upper) {
    return String(lower);
  }
  return String(lower) + '->' + String(upper);
}