main.cpp

// Source: https://leetcode.com/problems/summary-ranges
// Title: Summary Ranges
// Difficulty: Easy
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// You are given a **sorted unique** integer array `nums`.
//
// A **range** `[a,b]` is the set of all integers from `a` to `b` (inclusive).
//
// 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"
// ```
//
// **Constraints:**
//
// - `0 <= nums.length <= 20`
// - `-2^31 <= nums[i] <= 2^31 - 1`
// - All the values of `nums` are **unique**.
// - `nums` is sorted in ascending order.
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <sstream>
#include <vector>

using namespace std;

class Solution {
 public:
  vector<string> summaryRanges(vector<int>& nums) {
    int n = nums.size();
    if (n == 0) {
      return {};
    }

    vector<string> ans;

    long long start = nums[0];
    long long prev = nums[0];

    for (auto i = 1; i <= n; i++) {
      int num = i < n ? nums[i] : prev;
      if (num != prev + 1) {
        ostringstream oss;
        if (start == prev) {
          oss << start;
        } else {
          oss << start << "->" << prev;
        }
        ans.push_back(oss.str());
        start = num;
      }
      prev = num;
    }

    return ans;
  }
};