[LeetCode] 163. Missing Ranges

Given a sorted integer array where the range of elements are in the inclusive range [lower, upper], return its missing ranges.
For example, given [0, 1, 3, 50, 75], lower = 0 and upper = 99, return ["2", "4->49", "51->74", "76->99"].

Thought process:
  1. Get missing range from lower to nums[0].
  2. Get missing ranges in nums.
  3. Get missing range from nums[len - 1] to upper.

Solution:
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class Solution {
    public List<String> findMissingRanges(int[] nums, int lower, int upper) {
        List<String> ranges = new ArrayList<>();
        int len = nums.length;
        
        if (len == 0) {
            if (lower <= upper) {
                ranges.add(getRange(lower, upper));
            }
            return ranges;
        }
        
        if (lower < nums[0]) {
            ranges.add(getRange(lower, nums[0] - 1));
        }
        for (int i = 1; i < len; i++) {
            if (nums[i] > nums[i - 1] && nums[i - 1] != nums[i] - 1) {
                ranges.add(getRange(nums[i - 1] + 1, nums[i] - 1));
            }
        }
        if (upper > nums[len - 1]) {
            ranges.add(getRange(nums[len - 1] + 1, upper));
        }
        return ranges;
    }
    
    private String getRange(int lower, int upper) {
        if (lower == upper) {
            return lower + "";
        }
        return lower + "->" + upper;
    }
}

Time complexity: O(n).
Location: Philadelphia, PA, USA

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