This problem is similar to Missing Ranges and easier than that one.

The idea is to use two pointers to find the beginning and end of a range and then push it into the result.

The code is as follows, which should be self-explanatory.

1 class Solution { 2 public: 3     vector
     
       summaryRanges(vector
      
       & nums) { 4         vector
       
         ranges; 5         int left, right, n = nums.size(); 6         for (left = 0; left < n; left = right + 1) { 7             right = left; 8             while (right + 1 < n && nums[right] + 1 == nums[right + 1]) 9                 right++;10             ranges.push_back(getRanges(nums[left], nums[right]));11         }12         return ranges;13     }14 private:15     string getRanges(int low, int up) {16         return (low == up) ? to_string(low) : to_string(low) + "->" + to_string(up);17     }18 };