228. Summary Ranges

Problem

You are given a sorted unique integer array nums.

Return the smallest sorted list of ranges that cover all the numbers in the array exactly. That is, each element of nums is covered by exactly one of the ranges, and there is no integer x such that x is in one of the ranges but not in nums.

Each range [a,b] in the list should be output as:

• "a->b" if a != b
• "a" if a == b

Example 1:

Input: nums = [0,1,2,4,5,7]
Output: ["0->2","4->5","7"]
Explanation: The ranges are:
[0,2] --> "0->2"
[4,5] --> "4->5"
[7,7] --> "7"

Example 2:

Input: nums = [0,2,3,4,6,8,9]
Output: ["0","2->4","6","8->9"]
Explanation: The ranges are:
[0,0] --> "0"
[2,4] --> "2->4"
[6,6] --> "6"
[8,9] --> "8->9"

Example 3:

Input: nums = []
Output: []

Example 4:

Input: nums = [-1]
Output: ["-1"]

Example 5:

Input: nums = [0]
Output: ["0"]

Constraints:

• 0 <= nums.length <= 20
• 231 <= nums[i] <= 231 - 1
• All the values of nums are unique.
• nums is sorted in ascending order.

Problem Summary

Given a sorted integer array nums with no duplicate elements.

Return the smallest sorted list of ranges that exactly cover all numbers in the array. That is, each element of nums is covered by exactly one range, and there is no number x that belongs to some range but does not belong to nums.

Each range [a,b] in the list should be output in the following format:

• "a->b" , if a != b
• "a" , if a == b

Solution Approach

• Easy problem. According to the problem statement, use a cursor variable to accumulate and find consecutive intervals. Once a break occurs, output in the format specified by the problem. There are multiple output rules: ranges with arrows, single-element ranges, and empty ranges.

Code

package leetcode

import (
	"strconv"
)

func summaryRanges(nums []int) (ans []string) {
	for i, n := 0, len(nums); i < n; {
		left := i
		for i++; i < n && nums[i-1]+1 == nums[i]; i++ {
		}
		s := strconv.Itoa(nums[left])
		if left != i-1 {
			s += "->" + strconv.Itoa(nums[i-1])
		}
		ans = append(ans, s)
	}
	return
}